Semi is semi auto, it usually has a capped rate of fire, meaning you can only fire up to that rate. For example if it's cap rate is 15 bps you can only shoot up to the speed of 15 balls per second.
(stolen from a persons answer for another question to save time)Ramping is a feature in some electronic markers that automatically shifts the mode of fire from semi-automatic to fully automatic as long as the trigger is pulled at a specified rate. Ramping can be difficult to detect, both because it may not be consistent or easily observable by a human. Ramping modes can also be hidden in the software, such that a marker will fire in legal semi-auto mode when being tested, but in an illegal ramping mode by the player.
To counter the problems with enforcing semi-auto rules, some leagues allow a specific ramping mode. The rule specifies a minimum time between shots of 66 milliseconds (approx. 20 shots per second), and that no more than one shot may be fired for the first three trigger pulls. The rate of fire is enforced using a device called a PACT Timer, a standard firearms timing device that measures the time between shots. The following are common league specific ramping modes. Many high-end markers have these modes preset in their firmware:
PSP Ramping - Ramping kicks in after 3 shots; player must maintain 5 bps to remain firing fully automatic. Rate of fire cannot exceed 15 bps.
NXL Ramping - Ramping kicks in after 3 shots; player need only hold down the trigger to maintain fully automatic fire. Rate of fire cannot exceed 15 bps.
Millennium Ramping - Ramping kicks in after 7.5 bps; player must maintain 7.5 bps to maintain fully automatic fire. Rate of fire cannot exceed 15 bps.