Concept

Some empires may decide that the basic CCX/BCX/CAX are insufficient and choose to develop and deploy larger hulls. Rule (D70.17) hints at some alternatives. In some campaigns, the GM may opt to allow BCHX (X tech versions of an empire's command rating 10 BCH such as the Federation's BCG or the Lyran's BC/BCH), the DNHX/DNX/DNLX, and the BBX/BBHX.

Prerequisites

For any given hull type, the empire probably needs to also have the non-X version of such a design (this is campaign specific, but is the suggested solution).
You must develop each of the hull types and subtypes that you wish to use- the BBX, BBHX, and BBLX are three separate projects (ditto the DNX, DNHX, and DNLX).
You don't need to develop the full set- an empire might decide that the BCHX is all they want; they might decide that the BBLX and DNLX are good enough (and thus ignore the other two BB and two DN types).

Stacking

Each of these hull types is bleeding edge and may have shock type limitations. Their stacking is also greatly limited:

The BCHX is, like the non-X version, limited via D17.40 to at most one per squadron (and you can't have both a BCH and a BCHX). It counts against the allowed 4 or fewer SC3 X1 hulls per SQ.
The DNX (this includes the DNLX, DNX, and DNHX subtypes) is limited to one per SQ (and again, you can't have both an X1 tech version and a non-X1 tech version of the hull, nor may you have two or more of these per SQ). It counts against the allowed 4 or fewer SC3 X1 hulls per SQ, even though it isn't a SC3 unit. You may have both a DNX and BCHX in an X1 SQ.
The BBX (this includes the BBLX, BBX, and BBHX subtypes) is limited to one per SQ (and still precludes you from having both the X1 tech version and the non-X1 tech version of the hull, nor may you have two or more of these per SQ). It counts against the allowed 4 or fewer SC3 X1 hulls per SQ, even though it isn't a SC3 unit. Unlike the smaller designs, it also consumes one SC4 slot. You may have a BBX, DNX, and BCHX in an X1 SQ. Note that the SC4 slot is treated like any normal SC4 slot (so EFO tech plus two FFX would result in two command rating slots being used in the SQ)

Examples

These examples are legal:

BCHX, 3xCAX (or fewer), assorted SC4 X1 hulls to taste up to command rating limits
DNX, BCHX, 2xCAX (or fewer), assorted SC4 X1 hulls to taste up to command rating limits
BBX, DNX, BCHX, CAX (or none), assorted SC4 X1 hulls to taste up to command rating limits, less the one SC4 slot that the BBX use
For each of the above examples, one CCX may replace a CAX (so the "best" generic X1 SQ would often be a BBHX, DNHX, BCHX, CCX, and assorted SC4 X1 hulls)

These examples are not legal:

BCHX, BCH, assorted other ships (two BCH hulls)
DNX, DN, assorted other ships (two DN hulls)
BBX, BB, assorted other ships (two BB hulls)
BBX, BBLX, assorted other ships (two BB hulls)
DNX, DNLX, assorted other ships (two DN hulls)