This feature extends the use of the genericstable feature (FEATURE(genericstable)). Ordinarily, user addresses whose host part is listed in a special class defined by the GENERICS_DOMAIN mc macro (Section 4.8.1) are looked up in the generics table. Thus, if the generics table contains this rule:

news     news@news.our.domain

and if that special class contains the domain our.domain, only sender addresses of the form news@our.domain would be looked up, and addresses of a subdomain form, such as news@sub.our.domain, would not.

If you declare this generics_entire_domain feature, and if you also declare contents for that special class with either GENERICS_DOMAIN (Section 4.8.1) or GENERICS_DOMAIN_FILE (Section 4.8.1.1), subdomains are also matched. That is, with this feature declared, news@sub.our.domain would also match and be looked up.