The m4 nullclient feature (FEATURE(nullclient)) causes all mail to be sent to a central hub machine for handling. Part of what it does is to define the mc macro MAIL_HUB:

define(`MAIL_HUB', `hub')dnl

This also causes $H to be defined as hub. If MAIL_HUB has a value, and if LOCAL_RELAY ($R) does not, all local email is forwarded to hub. If LOCAL_RELAY is defined, it takes precedence over MAIL_HUB for some mail. See Section 4.5.7 for a description of MAIL_HUB and how it interacts with LOCAL_RELAY.

You should never use this macro directly because it might change in a future release of sendmail. Use the MAIL_HUB mc configuration macro instead.