If you have a memory intensive assignment, you can specify here that it should use a Gigabox slot. Gigabox slots are allocated on an organization level.
As you publish your assignment, the Propagate gigabox settings to the assignment settings option on the publish assignment dialog will show and will also show if you publish a new version and the gigabox setting for the working copy is different to that set for the assignment. See Modify Assignments for more information.
By default it is enabled but you can disable it if you choose.