Copy you edit in Design while a custom page or treatment is shown belongs to that page: the Text tab says so at the top. Every field you leave alone comes from the default page, so a page can change one headline and keep the rest.

A page's own copy lives in each language's content file, beside the default page's, and validation checks it the same way: overflow, glyphs, missing copy. An error in a page's own copy only stops that page's files.

Optimization tests

Treatments of the default page, tested against it on part of the traffic.

Store text

Name, subtitle, keywords and the rest of the listing, per language, held to Apple's limits.