Он разделяет разделы в исходном файле Yacc. например Смотри http://dinosaur.compilertools.net/yacc/
Names refer to either tokens or nonterminal symbols. Yacc requires token names
to be declared as such. In addition, for reasons discussed in Section 3, it is
often desirable to include the lexical analyzer as part of the specification
file; it may be useful to include other programs as well. Thus, every
specification file consists of three sections: the declarations, (grammar)
rules, and programs. The sections are separated by double percent ``%%'' marks.
(The percent ``%'' is generally used in Yacc specifications as an escape
character.)
In other words, a full specification file looks like
declarations
%%
rules
%%
programs
The declaration section may be empty. Moreover, if the programs section is
omitted, the second %% mark may be omitted also; thus, the smallest legal Yacc
specification is
%%
rules