Does /etc/security/limits.conf apply for each process or for each user?

Issue

• When applying some rules in /etc/security/limits.conf, user can be confused by their applied range.
• It says that user can control account's limit by specify the user or group name in it.
• Confusing part is whether some rules such as nofile, stack and data are applied to the user in whole or applied for each process which is created by the specified user.