A steelbook goes into accessory category under these circumstances:
- It is sold on its own
- It is sold with a game but not packaged together*
- It is included in a set
* There are instances where a game does not have a (brick and mortar) retail release, but is sold as an online exclusive such as those from GAME or sometimes with the "limited release" publishers where the "Pack" has no physical enclosure, but is a name attached to an online SKU. The "pack" then includes a sealed copy of a game, and a (most times) sealed empty steelbook, but there is no outer packaging to contain these items except for the shipping box from their warehouse.
A steelbook goes into the game category only when it is sold in retail packaging and has the game media inside of it, such as with PS2 or PS3 steelbook editions. In this case, the steelbook would not get a separate entry in the appropriate accessory category.
Example:
https://vgcollect.com/item/980Final Fantasy XII Collector's Edition is a steelbook release that is in the game category. There should not be a separate entry made in the PS2 Accessory category just for the steelbook, because it is part of the packaging.
And as hoshichiri says, you can put into the description of a steelbook, or a game release, that it was "sold by x website/company where they included x other items" or something to that effect.