What's the deal with annuals?

Should a title's annuals be created as a separate volume? If not how do they get listed within the existing volume?

I've seen some Annual volues and some volumes with regular issues and an annual listed with the annual also listed as #1 alongside the regular #1 issue.

I think Annuals should be a separate volume, but I'd appreciate some guidance please?

I've done them as separate volumes. We'd have to clean up the others at some point.