412(l)(6)(A) says that "This subsection shall not apply to any plan for any plan year if on each day during the preceding plan year such plan had no more than 100 participants."

412(l)(6)© says that "For purposes of this paragraph, all defined benefit plans maintained by the same employer (or any member of such employer's controlled group) shall be treated as 1 plan, but only employees of such employer or member shall be taken into account."

Has this ever been clarified anywhere exactly what (6)© means?

Here is my situation:

Company A has 150 employees. They maintain a DB Plan that has 60 participants.

Company A is a member of a controlled group with company B. They qualify as QSLOBs if that matters. Company B has 500 employees. Company B maintains a DB plan with 450 participants. None of Company A's employees participate in B's plan, and none of Company B's employees participate in A's plan.

Does the DB Plan of Company A have over 100 participants for DRC purposes?

Dividing related employers into QSLOB's is only used for coverage and nondiscrimination testing, so I don't think it has any bearing here. That being said I say Company A definitely has over 100 participants for 412(l) purposes.