Explanation
The drift matters when people talk about workflow. A team saying they make reels almost always means they make one 9:16 video and put it on Instagram, TikTok and YouTube Shorts, because the same file works on all three. That is accurate as a description of production and misleading as a description of distribution, since each platform reads a different text field and applies a different set of rules to the same upload.
What distinguishes Instagram's version in practice is that it sits in a feed that also contains photos and posts from people you actually follow, so the competition for attention is different from TikTok's, where everything in the feed is a video from a stranger. Reels tolerate a slightly slower opening as a result, though not by much and not reliably.
The common mistake
Treating reel as a synonym for a specific edit. It describes a slot on a platform rather than a style, so a talking head, a slideshow and a product demo are all reels, and advice about what reels need is usually advice about one of those three.