For details of the what is allowed, you can view the terms of service.

In general, you can make any configuration changes on your slab that you like, so long as you don't interfere with the operation of DataSlab, or with the operation of a slab owned by another user. So you can install software, remove software, configure services, transfer files, etc. You can use your slab for commercial purposes, or just for fun, or somewhere in between. You can add user accounts, send and receive email, host web sites, host game servers, whatever.