Meaning of "." (dot) in netfilter

[Ross Boylan <rossboylan@xxxxxxxxxxxxxxxxxx>]

https://wiki.nftables.org/wiki-nftables/index.php/Simple_ruleset_for_a_home_router
include a number of rules like

        ip protocol . th dport vmap { tcp . 22 : accept, udp . 53 :
accept, tcp . 53 : accept, udp . 67 : accept}
with possibly 2 different uses of ".", namely 'ip protocol .'  and 'udp . 67'.
What do those "."'s mean?

I've looked in available documentation, but can't tell.

Thanks.
Ross Boylan