Where is $<tag>$ defined?

In 3.7.6 Performing Actions before Parsing and 3.7.7 Freeing Discarded 
Symbols, $<tag>$ is referenced. I can't find any definition of this. I 
do find definitions of other $<other>$ constructs, but not of tag. I 
assume that 'tag' has the same meaning as in 3.7.5 Syntax of Symbol 
Declarations.

In particular, 3.4.6 Actions makes no mention of this format, and 
instead provides (what I assume) are all the possible alternatives to 
referencing values from items in the RHS of a BNF statement, and 
assigning values to the LHS BNF name.

Where is this format defined?

thanks
art