EtherPad Private Network Edition (PNE) uses a floating-seat licensing plan. A floating-seat license limits the number of active users. A user is considered "active" if he or she has used PNE in a 24-hour period.
A "user" is determined by a browser cookie, so a single human being using two computers or two browsers on one computer will be counted as two users.
Users can be members of your organization, or outside clients or contractors with whom your organization collaborates.
A good rule of thumb for deciding how many users to buy for your license is:
For example, if there are 100 people who will actively use EtherPad at your organization, and on any given day your employees conduct meetings with 20 people outside of your organization, then a 120-user license makes the most sense.