Common naming conventions and internal property names that are still used in the API:

cid : Refers to your account ID, accounts are used to be called campaigns.
rupid : Refers to contact unique ID, mostly it is a phone number
action : Refers to an action or texting project, actions may represent functionality not limited to texting
gid : Refers to a contact group ID
proxy : Refers to a virtual proxy number to send from
ctime : Refers to creation time in milliseconds since Epoch
mtime : Refers to last update time in milliseconds since Epoch

Note: Only documented properties are guaranteed to be present in API responses, some properties may be marked as deprecated but any undocumented property that appear in responses can be removed without any notice.