1Password also thinks site1.example.com and site2.example.com are the same.
For a password saved from foo.contoso.com:
First option - "Fill anywhere on site" - this means it will fill on foo.contoso.com and bar.contoso.com
Second option - "Fill only on the specified hostname" - will only fill on foo.contoso.com
Third option "Never fill" - written on the tin.
edit: Ooops, just realised you were responding about 1Password, sorry.