What determines that flag value in a PST file is how it was created. It
can't be changed after the PST file was created. If the PST was created as
an Outlook 2003 or later PST it's Unicode enabled. If created as compatible
with earlier versions of Outlook it is not Unicode enabled.

well this is what I also originally thought

but situation at my computer was that even PST files that were UNICODE (as
seen from the same computer from same Outlook under different Win profile)
showed ANSI only support.

I think that you can have more PSTs in one Outlook mail profile each one
having different support and different value of this flag. Because store is
PST.

So I had corrupted win profile somehow and wondered how the client really
determines the value because it showed ANSI for all PSTs, even for those I
now normally use UNICODE and I did not modify at all, just login in under
different win profile.