In the ‘Key pattern’ tab in Preferences, one can set the fields used in auto generation of BibTex labels. The pattern can be set for each of the standard entry types.
The pattern can contain any text you wish, in addition to field markers that indicate that a specific field of the entry should be inserted at that position of the key. A field marker generally consists of the field name enclosed in square braces, e.g. [volume]. If the field is undefined in an entry at the time of key generation, no text will be inserted by the field marker.
Several special field markers are offered, which extract only a specific part of a field. These are listed below. Feel free to suggest new special field markers.
Special field markers:
auth]: The last name of the first
        authorauthors]: The last name of all
        authorsauthorLast]: The last name of the last
        authorauthorsN]: The last name of up to
        N authors. If there are more authors, “EtAl” is
        appended.authorsAlpha]: Corresponds to the BibTeX
        style “alpha”. One author: First three letters of the last name.
        Two to four authors: First letters of last names concatenated.
        More than four authors: First letters of last names of first three
        authors concatenated. “+” at the end. authIniN]: The beginning of each
        author's last name, using no more than N
        characters.authorIni]: The first 5 characters
        of the first author's last name, and the last name initials
        of the remaining authors.authN]: The first N characters of
        the first author's last nameauthN_M]: The first N characters
        of the Mth author's last nameauth.auth.ea]: The last name of
        the first two authors, and “.ea” if there are more than
        two.auth.etal]: The last name of the
        first author, and the last name of the second author if
        there are two authors or “.etal” if there are more than
        two.authEtAl]: The last name of the
        first author, and the last name of the second author if
        there are two authors or “EtAl” if there are more than
        two. This is similar to auth.etal. The
        difference is  that the authors are not separated by “.”
        and in case of more than 2 authors “EtAl” instead of “.etal”
        is appended.authshort]: The last name if one
        author is given; the first character of up to three
        authors' last names if more than one author is given. A
        plus character is added, if there are more than three
        authors.authForeIni]: The forename initial 
        of the first author.authorLastForeIni]: The forename initial 
        of the last author.[auth...] markers will use the editor(s)
    (if any) as a fallback. Thus, the editor(s) of a book with no
    author will be treated as the author(s) for label-generation
    purposes. If you do not want this behaviour, i.e. you require a
    marker which expands to nothing if there is no author, use
    pureauth instead of
    auth in the above codes. For example,
    [pureauth], or
    [pureauthors3].
    edtr]: The last name of the first
        editoredtrIniN]: The beginning of each
        editor's last name, using no more than N
        characterseditors]: The last name of all
        editorseditorLast]: The last name of the last
        editoreditorIni]: The first 5 characters
        of the first editor's last name, and the last name initials
        of the remaining editors.edtrN]: The first N characters of
        the first editor's last nameedtrN_M]: The first N characters
        of the Mth editor's last nameedtr.edtr.ea]: The last name of
        the first two editors, and “.ea” if there are more than
        two.edtrshort]: The last name if one
        editor is given; the first character of up to three
        editors' last names if more than one editor is given. A
        plus character is added, if there are more than three
        editors.edtrForeIni]: The forename initial 
        of the first editor.editorLastForeIni]: The forename initial 
        of the last editor.firstpage]: The number of the
        first page of the publication (Caution: this will return 
        the lowest number found in the pages field, since bibtex 
        allows 7,41,73--97 or 43+.)keywordN]: Keyword number N from
        the “keywords” field, assuming keywords are separated by
        commas or semicolons.lastpage]: The number of the last
        page of the publication (See the remark on 
        firstpage)shorttitle]: The first 3 words of
        the titleshortyear]: The last 2 digits of
        the publication yearveryshorttitle]: The first word of
        the title, discounting ‘the’, ‘a’, ‘an’.A field name (or one of the above pseudo-field names) may optionally be followed by one or more modifiers. Modifiers are applied in the order they are specified.
volume field is not set.
        If you haven't defined a key pattern for a certain entry type, the Default key pattern will be used. You can change the default pattern - its setting is above the list of entry types in the Key pattern section of the Preferences dialog.
The default key pattern is [auth][year], and this could
    produce keys like e.g. Yared1998 If the key is not
    unique in the current database, it is made unique by adding one
    of the letters a-z until a unique key is found. Thus, the
    labels might look like:
        Yared1998
        Yared1998a
        Yared1998b
    
After the key pattern has
    been applied to produce a key, you can choose to have the key
    generator search for occurrences of a specific regular
    expression, and replace it with a string. The regular
    expression and replacement string are entered in the text
    fields below the list of key patterns. If the replacement
    string is empty, matches of the regular expression will simply
    be removed from the generated key. For instance,
    \p{Punct} or [:/%] can be replaced
    by nothing to remove unwanted characters from the key.
    This may be useful when naming PDFs according to BibTeX keys.