Dear colleagues-modders, I made the mistake. I have 2 different .biq's with different settings, so in 1 of them Captured is "tradeable", in another one - "Heir". But not in 1 single .biq simultaneously. I experimented with .biq's more than month ago (I left it to be done for the future - according to my dev notes yesterday night), so I just forgot. My apologies for misinforming you.