Solved!

Did a search, looked around, found nothing

Say you have an escort, and the ship in question, ship A, can be upgraded to ship B. But ship B has "bxxx" put in the Availability and AppearOn fields. I suppose one will not be able to upgrade the escort before bit xxx is set, but as the Nova bible puts it,
"Availability Control bit test expression. The player will be able to purchase
this type of ship when the expression evaluates to true. Leave
blank if unused. Depending on the configuration of other flags,
the ship might appear in the shipyard but not be able to be
purchased if its Availability evaluates to false.

AppearOn Control bit test expression. Ships of this type will not show up
in dude resources if this expression evaluates to false. "

No mention of escorts
So can you upgrade if the bit is not set?

This post has been edited by Pace (haldora) : 12 January 2005 - 07:25 AM

Okay, since no one replied, and since I just had enough time, I tried out with a dud plug.

It appears that the AppearOn field doesn't influence at all what upgrades are available for escorts: if you put "bxxx" inside it, it won't change anything. You still can't upgrade.
It is the Availability field that controls that. So if you don't have "bxxx" set, and it is in the availability field, then you will not be able to upgrade.

So basically, you can see upgraded ships elsewhere (i.e. in düde and përs resources ), but cannot buy them nor upgrade your escorts to them.

Could be used to do some pretty interesting things, I reckon (goes off to implement it in Arpia 2 as originally planned)