It's the ID of the cast record. Similar to the credit_id in TV credits, but less polished. cast_id references the unique and particular item in the credits. Imagine an actor had two roles in the same film. You can use cast_id to keep track of them individually. Another example would be, imagine someone deletes but then re-adds an actor. These would have different ids and you would know that something changed.

TV's version is much more complete, since there's a full credit lookup method and what not. This will eventually be migrated to on movies but it's a sizeable effort and not something I have plans for in the immediate future.

Cheers.

I am guessing it isn't used properly very much? (Purely in relation to films.) e.g. Nutty Professor has Eddie Murphy doing two roles, clumped together and given the cast_id of 1.

For Coming to America, there are 4 roles done by Murphy, all with cast_id of 1 as well.

It is used, in every instance where it applies... Coming to America isn't a good example since there is only a single entry for Eddie Murphy.

A good example in this case would be if I deleted Eddie Murphy and then re-added him, he would get a new id. This is also how changes are tracked when multiple entries for the same person exist. For example, if you changed a character name.

Ideally though, shouldn't Eddie be entered 4 times for the 4 roles he plays? So each role is a separate row? In regards to users entering film details.

We don't have a firm policy around that. I'm actually more inclined to prefer the grouped credits, otherwise it can get pretty crazy. Consider The Simpsons for example.