I have never seen this explained in any official Blender documentation to date.
Not sure this text is so helpful... for what its describing its very verbose ... and could possibly be a much shorter paragraph.
However, only one type of mesh component can
be selected at once—the one corresponding to the leftmost highlighted mode button.
This is a bit misleading - you can for example - border-select, to select multiple components 'at once'.
Perhaps text could read:
Using the buttons you can also use more than one selection mode at a time by
:kbd:Shift-LMB clicking the buttons.
This allows you to easily select different kinds of elements without having to switch modes each time.
My description may be verbose, but it is not misleading: even with border-select, circle-select or whatever, you can still only select one type of component at once.
Perhaps a more detailed example is needed to illustrate. Start a new Blender document. Select the default cube. Go into Edit mode. Deselect everything. Enable Vertex+Edge selection mode. Look at the top face, pick one of its border edges, and go round from there: RMB on the first edge, shift-RMB on the second edge, shift-RMB on the third edge, and finally shift-RMB on the fourth edge.
If you were selecting edges, you would have four edges selected by now. But in fact you are selecting vertices, so you end up with just two vertices selected.
I can provide examples for other mode combinations, if you like.
With Vertex+Face modes both enabled, You can border select over a face-center and an isolated vertex. Circle select with a single click too.
In general, think this may be a case where a user needs to try out the option to fully understand the impact. We can explain exact behavior of different combinations ... but in practice I think we're better off giving a short explanation, and users can learn *exact* behavior or selection flushing, mixing different modes... etc by using.
With Vertex+Face select mode, you can shift-select up to two faces on the default cube and still be able to shift-deselect them individually. Once you select more than two faces, however, they all become selected, and you can no longer shift-deselect any of them.
I agree this doesn’t agree with my explanation. But it doesn’t agree with yours, either. Bug?
Thanks for that quick fix.
But it now works more like my description.
For example, in the default cube with nothing initially selected, enable vertex+face selection mode. RMB one face, then shift-RMB an adjacent face. Now shift-RMB on either of the selected faces, and you see that the selected/active highlight disappears from both faces, and you end up with just two vertices selected.
If faces were being selected, then both faces would not be deselected.
@Lawrence D'Oliveiro (ldo), right - this is caused by selection flushing with mixed mode selection.
However Im not sure its useful to explain selection flushing behavior in detail. These things you can notice when you use.
OTOH, some short explanation could be OK.
To pick an unrelated but similar case:
cursor navigation when holding control has some logic which would take a paragraph
or so to explain - logic for skipping words, also used for double-click selection.
We could explain the rules here, but in this case, like selection flushing, I think its not really all that helpful, users can grasp the basic idea - and will notice selection flushing works a bit different when mixing modes - and thats fine