UML14 — Ambiguous example of a local action on a Lifeline in Figures 334, 335

Page 415-6 Figures 334,335 use a rectangular box with the text "DoSth" in it. The meaning of this symbols is not explained in the corresponding section, nor in Table 14 graphical nodes in Sequence Diagrams.
In ITU Message Sequence Charts this would mean an informal action, local to the Lifeline.

The syntax and semantics of local actions is the key issue in "executable sequence diagrams", and in proper alignment of semantics between Interactions, Activities and State Machines (and Actions).

As a minimum, Figures 334, 335 need to be explained, and table 14 updated.
It would be better to illustrate formal use of Actions.
Ideally, Interactions will benefit from a data sublanguage based on Actions, in order to have a capability to fully specify flows of data between Lifelines:

• message parameters (including composite values)
• local attributes (storing data in Lifeliens; in data structures like lists, sets, tables, etc.)
• identities of objects (input and output pins for actions)
• actions (manipulations on data, access to data structures and composite values)
• proper usage of data in guards and state invariants
• proper usage of data in loop expressions

see above