A review of positions is done at some point during bargaining if it looks like it's heading towards a strike (assuming a strike vote and everything leading up to it was undertaken) and positions are identified that are needed to keep the business running. This is negotiated between the employer (GC) and the union (PIPSC, PSAC, et al) as the employer would like to see everyone working, the union wants everyone off ... But the business needs to run. Once the 2 parties agree who's getting designated, you get called into a meeting with the employer & the union and you're given a form that you're being designated that you have to sign. I don't recall what happens if you refuse to sign (or if you even can?)

People that get designated are people like border guards, immigration officers, swaths of CS's (gotta keep the computers running), financial & purchasing officers. Basically a skeleton staff if you will to keep the GC running day to day