My understanding is that 8-C-4-a secondary vacancies are about keeping the training pipeline full.

For example, there are 10 advertised vacancies for DEN 787 CA. Let's say that 10 787 captains bid over to DEN from SFO. Those pilots are laterals and therefore don't take up a training slot. In this case, UPA 8-C-4-a allows the company to make additional "secondary" awards up to the original number of advertised vacancies so that the allotted training slots are filled.

So in this example, up to 20 DEN 787 CA bids could be awarded: the 10 laterals from SFO and up to 10 additional awards to fill the training pipeline.