Tortoise Merge options

Tortoise Merge options

I don't fully understand the difference between the three Tortoise SVN
merge options. My understanding is as follows:
Merge a range of revisions - use to apply changes to the Trunk to a Branch
when the Branch is incomplete.
Reintegrate a branch - use when a branch is complete.
Merge two different trees - not sure
I have read this:
http://tortoisesvn.net/docs/release/TortoiseSVN_en/tsvn-dug-merge.html. I
have also read a few questions on here e.g. this: SVN Merge a range of
revisions vs. reintegrate.
It appears to me that they all do the same thing, but I am obviously
wrong. Why are there three options?
The answerer to the question in the Stackoverflow link suggests that
Tortoise SVN uses a different approach when identifying the difference
between a branch and the Trunk i.e. it does not use revision numbers (it
just looks for differences between the files).
The documentation to me suggests that "Merge two different trees" is used
instead of "Merge a range of revisions" if there are no version numbers
i.e. simply look at the head. Surely you can do this with: "Merge a range
of revisions ".