You can do various sections provided you have the necessary information to continue.
1. Functional Description
2. Chapters for the Manual (Including Service Manual)
3. Offline Software such as configuration, tags, comments and logic.
I recall writing software for a future project whilst on-site commissioning at another client.
I also recall writing chapters for a project where, in hindsight would have been impossible to work at home for constantly seeking information only obtainable at the office.