One of the most important decisions chairs should make about reviewing is the structure and the hierarchy of reviewers.
The term "reviewer" is used by different communities in different ways. For this reason, this term is rarely used in EasyChair. In EasyChair there are two main kinds of reviewers: program committee members (aka PC members) and subreviewers (called external reviewers in some communities).
We receive a lot of questions related to differences between PC members and subreviewers.
The main difference between PC members and subreviewers is that PC members already agreed to reviewing for the conference, so chairs can assign submissions to them. Subreviewers cannot be assigned and are normally used by PC members to delegate reviewing. To do so, a PC member can send a review request for a specific submission to a subreviewer, who can accept to reject this request.
This is a simple summary of differences between PC members and subreviewers: