I popped into my local shop inquiring about buying a threaded headset for a used bike frame I'm upgrading. The fork is threaded and has a 1 1/8 diameter. The guy told me it would be better to bring the frame and fork in. He gave me the impression there was more to the purchase than simply fork diameter and threaded/threadless compatibility. I thought headsets were a cut and dry topic with only two factors to consider: threaded/threadless, and fork/steerer diameter. When the mechanic/store owner was talking to his colleague/son about my frame (he removed my headset cups last week and recalls the frame) he mentioned integrated headsets. There are some language barriers present, so he wasn't able to fully articulate everything, but he basically told me there was more to this than what I thought.

The bike is a mid 90s Giant mountain bike. I thought integrated headsets were strictly a threadless topic. I almost bought a frame that required one, which is something I looked into afterwards.

The guy seems pretty honest. What am I missing? All threaded headsets I find online appear to differ only in regards to steerer/fork diameter, and nothing else.