These are permanent additions or changes made to a building by a lessee at his or her own expense that may not legally be removed.   If you have made these types of improvements to your office it will be your responsiblity to insure these improvements.
For example if you added some permanent cabinetry to your office space that were not provided by the landlord, you should be insuring these improvements under tenants improvements.  Your landlord will expect you to have your own coverage for this.
Related: Tenant Insurance (Westland)