UML14 — UML RTF 1.4 Issue: Arrowhead semantics in collaboration unclear

The semantics of the stick arrowhead is described as:

[p 124, notation] Flat flow of control. Each arrow shows the
progression to the next step in sequence. Normally all of the
messages are asynchronous.

[p 128, notation] stick arrowhead: flat flow of control
(normally asynchronous).

What is a "flat flow of control"? How is it different than an
asynchronous operation or signal? It is ambiguous to say that it
is "normally" asychronous. How does the user tell whether it is
or isn't in any particular case?

It is more confusing when compared to the descriptions for
half-stick:

[p 124, notation] Asynchronous flow of control. Used instead of
the stick arrowhead to explicitly show an asynchronous
communication between two Objects in a procedural sequence.

[p 128] half stick arrowhead: an asynchronous operation invocation

How is a "flat flow of control" different from a "procedural
sequence"?

This topics has been very confusing for the agent modelers, who
are using sequence diagrams to model agent protocols.

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