As far as I know....
Master couplers are "normally" outside sample sets and not included in the combinations provided by the sample set itself. They (and "usually" all of the sample set stops) may be captured by HW's master combination system (pg 98 in the HW user Guide). The master scoped combinations (pg 101) can be used to provide similar functionality to the divisional pistons often provided by sample sets and these too can include the master couplers.
The "normally" and "usually" are included because sample sets can stop master couplers being available.
Hope this helps.
Iain