The exemption in plain English
If you are directly employed (on a contract of service, not for services) by the entity that owns or occupies the premises you're guarding, and you are not doing door supervision at licensed premises, you may be exempt.
Where it commonly breaks
You work for a facilities management company that supplies you to a client site — you are contracted, not in-house.
The site has a Premises Licence and you handle the door on entry — Door Supervision applies regardless.
Why most sensible in-house teams get the licence anyway
Portability: your career doesn't stall the moment you leave.
Insurance cost: many corporate insurers require licensed guards even where exemption technically applies.