TRANS13 — Shared/unshared transactions?

the merged spec (with the messaging changes) talks about shared and unshared
transactions. It also obliges the client and server side to perform
certain actions when the state of the transaction is incompatible with
the transaction policy of the target object. However, this begs the questions:

1) How does a client decide whether to create a shared or an
unshared transaction?

Currently, all the client can do is to call begin(), so what
kind of transaction is created when the client does that?

2) How does the server learn about what kind of transaction was
created by the client so it can perform the required checks?

Right now, there is neither an interface on the coordinator
nor is there anything in the transaction context that would
allow the server to find out.

It appears that there must be a way to:

1) for the client to control what kind of transaction to create

2) for the server to find out what kind of transaction was
created by the client.

Issue was addressed by the proposal for issue 3425, so it is being closed without action.