Some part is usually treated as vested for time already served. The share certificates are legended and, where the agreement calls for it, held in escrow, and the shareholders’ agreement is checked so the two documents do not contradict each other.
Investors ask for this at almost every seed round, because they are buying the team’s continued work and not only its past work, and a co-founder who joined later is often asked to accept it by the others. Doing it before the round, on the founders’ own terms, is cheaper than doing it under a term sheet. The issuance itself also needs a prospectus exemption under Ontario securities law; for shares to founders, employees and consultants of a private company that is usually one that requires no report, and the kit confirms which applies. Anything that needs a filing, a valuation or a tax opinion is scoped separately, by the same firm, before work begins.