domains code query

*
domains.c contains the followng snippet in domain_in():

   else* *if* (my_extra->domain_type != domainType)    domain_state_setup(my_extra, domainType, false,
    fcinfo->flinfo->fn_mcxt);
 


We were just looking at this code (in the context of implementing enums) and wondered when this case might arise. 
Would it be when more than one domain is used in a table row? Or are we smarter than that?

Putting it another way, how often are fcinfo/flinfo created and reused?

TIA

cheers

andrew