I agree with matejcik’s point 1 to 3 and especially with point 4.
The mandatory flag (or optional-flag) makes much sense to me.

> -----------
> 
> In general, the standard is trying to be very space-conservative,
> however is that really necessary? We would argue for clarity and ease of
> use over space constraints. We think more straightforward approach is
> desired, although more space demanding. What are the arguments to make
> this as small as possible? If we understand correctly, this format is
> not intended for blockchain nor for persistent storage, so size doesn’t
> matter nearly as much.

I don’t see any reasons why space would be an issue.

HWWs probably can’t handle PBST natively since it is not optimised for
presenting various informations in a signing-verification.

A single stream-in of a PSBT through USB (or similar channel) will not work in
many cases since HWW come often with very restrictive RAM constraints.

Furthermore, I forget to mention in my last mail, that registering (or defining)
a mime-type for PSBT would probably a great usability feature.
(Send PSBT by email/messanger and with dbl-click to open feature, etc.)


/jonas