We rank candidates in the pool using a points-based system called the Comprehensive Ranking System (CRS). When we hold Express Entry rounds of invitations, we

  1. choose the type of round we’ll hold
  2. decide the number of candidates we need to invite
  3. identify the highest-ranking candidates from the pool who are eligible for the chosen round type
  4. invite these candidates to apply for permanent residence

Read more here: https://www.canada.ca/en/immigration-refugees-citizenship/services/immigrate-canada/express-entry/submit-profile/rounds-invitations.html