A platform defines the confines of the game world.
A game platform comprises all the things required to play the game.
For example, a newspaper crossword is the paper as well as the pencil the player uses. Depending on the content, one could also include reference materials as part of the game.
An online escape room would comprise the computer on which it's played, including the keyboard and mouse. It may also include the pen-and-paper a player could use to assist them.
While pen-and-paper may seem like a small detail, it's an important part of the platform. This is especially true in controlled environments, like a live escaper room, where this material may not be provided. The platform for such a room is limited to strictly what a player finds in the room, less objects marked with stickers saying "don't touch".
If you have any questions, need an example, or want clarification, then let me know. Ask on Discord or Twitter.
Assume everything in this reference is a working draft, there's prone to be some mistakes and inconsistencies. I figure it's best to publish and get feedback rather than write for years in secret. The terms will change, the structure will shift, and the bugs will be chased out. It'll take a while.