Vesting conditions determine both how long the expense is spread and how many awards are expected to vest. IFRS 2 distinguishes sharply between non-market conditions (which affect the number of awards expected to vest and require periodic revision) and market conditions (which are priced into the grant-date fair value and never revised afterwards) — IFRS 2.19 and IFRS 2.21.
A service condition requires the counterparty to complete a specified period of service. It is the most common condition in employee share plans.