Xc (Capital survey Instruments) was one of several tech rules introduced by 3DG (3rd Edition Design Group), the bunch who consolidated the Unified Tech Manual.
The next project after that was the Unified Rules Manual (URM) which was intended to amalgamate the various 3rd Edition rules sets into one place, and to fix a number of balance issues, or conflicts with the materials like In Death Ground.
The problem here was that the Hun Class survey cruisers in IDG/ISW4 just dont make any kind of economic sense. Hence the development of the Xc to allow larger units to at least approach the the surveying/cost capabilities of the standard early game unarmed EX. .
The changed Hun came out something like this
Hun Class CA(S) (AC) (AM2) XO:10 54 Hull Spaces HT 12 PV: 48.0
[1] S1x9 Ac2x12 H (BbL) (IcIc) H (IcIc) (IcIc) Wa Qs Wa Xr Mg Qs Dxz ?k ?3 Z2 Dxz Xc Qs (IcIc) [4/ 4]
RCP:54 MCP:21 MSP:200 End: 50 Cloak Trg: 1 Atk: +0 Bmp: +6 Tem: +0 Cost: 1890.7 / 283.61
45 HTK S1x9 Ac2x12 Wax2 Dxzx2 Mgx1
Huns were still far from optimal, but at least they made a little more sense after the redesign. There was an ongoing debate about adding a second Xc (which would have improved the survey capabilities, while retaining the "undersized CA" of the blurb).
Steve, like a number of others on this list, was part of 3DG, and tended to put tech changes into SA once they had been agreed on by the group (or ocasionally before, beacuse he thought they were a good idea for his campaign eg Mine Racks)
Unfortunately many of these ideas were never formally published due to the demise of 3DG.