[Exim] Syntax clarification @@lsearch*

I have the following line in my v4.14 ACL:

deny senders = @@lsearch*;/name/of/the/file

which points to a file with format:

domain1    test1:test2
domain2    test1:test2
domain3    *

I know the deny statement works because I have tested it but I have
searched every relevant chapter of the docs and can't figure out what
the "@@" means.


Peter