49 CFR §37.133

1. (a)This part does not prohibit the use of subscription service by public entities as part of a complementary paratransit system, subject to the limitations in this section.
2. (b)Subscription service may not absorb more than fifty percent of the number of trips available at a given time of day, unless there is non-subscription capacity.
3. (c)Notwithstanding any other provision of this part, the entity may establish waiting lists or other capacity constraints and trip purpose restrictions or priorities for participation in the subscription service only.