The @string entry is special because it defines some kind of template. This example is taken from the BibTexing document by Oren Patashnik and it describes how to produce nearly identical title fields for different entries:
@STRING( WGA = " World Gnus Almanac" ) @BOOK(almanac-66, title = 1966 # WGA, . . . @BOOK(almanac-67, title = 1967 # WGA,The two string in both title values are concatenation by the '#' character. This character has to be surrounded by whitespaces.
It also has to be mentioned here that it is only possible to define one parameter using the string entry. So the following entry is invalid:
@String(mar = "march",
apr = "april")
This has to be written like this:
@String(mar = "march") @String(apr = "april")