A Baseball card signed by a player, a film poster by an actor, or a figure by a designer is a Database Item in its own right and we add them as a Subvariant to the not-signed item. Here is an example -
We do not differentiate between the position of the signature, the color of the pen or what is written and there is only on Subvariant entry for any logical signee (so for a character his designer, an actor that played the character or somebody else meaningful). IF there is a limited run of signed items with their own branding (say in the form of a sticker) then those will get added as a separate Subvariant.
More about how to add Subvariants here.