The adjectival is a noun that modifies another noun. In simple words, the noun give a description about where the other noun comes from, what it is for, etc. You can find many of them in compound nouns.

For example:

Pencil case: The case is used to save some pencils in.

Rubbish can: The can is employed to throw away the rubbish.

Video type: The type is for recording videos, not only audio.