UML14 — Figure 395 requires a lot more explanation

Figure 395 requires a lot more explanation. The abstract syntax claims that
the effect of a transition be expressed as an Activity and so I suppose this
is what this diagram represents, but I don't recognise the rectangle
notation from activities. It's also not clear whether the arrows represent
flows or transitions - if fact they can't be either, because some of the
arrows start on states and end on actions. It also isn't clear whether there
are any rules about the construction of these transitions; for example, I
assume that there can only be one signal receipt and that it has to be the
first symbol encountered, but that isn't stated. There may be an explanation
that I missed, in which case it should be placed nearer the figure, or an
appropriate reference inserted. The various symbols should also appear in
the diagrams section.

see above