Changeset - a unit or set of items?

I came across this post on forums, and thought it is worth to elaborate a bit.

In a nutshell - when you merge from path X to path Y, and specify certain changeset to merge, will all files in the changeset will be merged from X to Y? The answer: no, only files that both under X and in the changeset will be merged.

From one point of view, it is not very logical, since when you select a changeset you probably trying to merge all changes in it. On the other hand, you explicitly specify source and destination path and expect to merge items only under path, so if you look at changeset as a specification of versions to include in the process, rather than as a "container" unit of sorts, the results are logical.

Another changeset usage that I have found to be puzzling for some users, is specifying changeset for "Get specific version". Again, many people assume that changeset here behaves as a unit, and expect to get only files included in the changeset. But changeset instead behaves as mere date/time specification, and therefore versions relevant for that changeset timestamp will be retrieved for all files in the specified path.

Overall, those nuances are pretty important to be aware of (especially if explained to the users in advance).