In the credits of video games there are often separate headings for "Additional Programming", "Additional Design" or "Additional Art"- often containing the names of people who work in other departments. What is the need for this separation and how do "Additional" developers differ from the "regular" ones. Does this imply that people from other departments do design/programming/animation as well?
In the movie business, professionals often belong to guilds (actors, writers etc.) who collectively bargain for exactly how their members are compensated, including credits. The rules are quite comprehensive. On the other hand, there are no such fixed rules for the games industry. Instead, some companies create their own rules on how to write credits and who deserves them. Obviously, this leaves a lot of people unhappy.
There are efforts to standardize the credits; IGDA have a group working on a game crediting guide. As of writing this is still in draft form and is supposed to be a "recommendation" only, but it can give us some clues on what those credits might mean. Here's the relevant section:
1-3. For non-Leads, if the contribution consists of less than 40% or eight months (whichever is least) of the project’s total workdays in development, then the credit may be listed in a lower tier, e.g. “Additional Programming.”
But none of this is industry standard, so yes it could also mean whatever they want.
It means whatever the company wants it to mean.
There are no mandated crediting standards in this industry, and so just as the precise meanings of job titles differ from studio to the studio, the precise meanings associated with credits terminology vary as well. In fact, it can often vary from product to product, even within the same studio.
The "additional" prefix could mean a variety of things:
As you surmise, it could mean the people in question have "official" job titles other than what they are being credited for.
It could mean the people in question were contractors, as opposed to full-time employees.
Similarly, it could mean that the people in question were from another division of the company (particularly in the case of large companies that actually own multiple independent studios).
It could mean the people in question did not work full-time on the project, but were perhaps called in to do some fire-fighting during critical milestones or similar.
It could mean the people in question were not full-time employees at the time the credits were finalized (typically when the game is considered "gold," or shippable), but worked sufficiently long on the project prior to their departure to still receive some form of credit.
It could mean the people in question have names longer than 15 letters.
That last one is unlikely, of course, but I include it simply to underscore how arbitrary credits can be.