Most commercial buildings hold more than one occupancy, and Section 508 gives three ways to handle the mix. Accessory occupancies (508.2) are small subordinate areas that follow the main occupancy. Nonseparated occupancies (508.3) skip fire separation between uses — but the most restrictive requirements then govern the entire building. Separated occupancies (508.4) use fire barriers or horizontal assemblies at the ratings in Table 508.4, letting each area follow its own rules.
The trade-off is real: nonseparated simplifies detailing but can push the strictest occupancy's sprinkler, egress, and height/area limits onto everything. Separated adds rated construction but keeps each use on its own requirements.
A common mistake is choosing nonseparated to avoid rated walls, then missing that the whole building has now inherited the most restrictive occupancy's requirements. Reviewers look for the 508 approach to be stated explicitly, and where it's separated, the Table 508.4 ratings shown on the plans.
This guide describes the model code for general understanding and is not a substitute for the adopted code and amendments enforced by your local authority having jurisdiction. Verify all figures against your jurisdiction's codes of record.