One if the two Well Controls with Lift Mechanism trying to maintain the manually specified value of the flowrate.
In physical
Liquid control is most common for producing wells with SRP and ESP.
The mechanical pump is trying to maintain the preset flowrate but it only succeed when the wellbore fluid level is abiove pump location.
Once the fluid level lowers to the pumping chamber the pump efficiency deteriorates and the flowwaret reduces while fluid level and correpsonding BHP are getting stabilized.
except the case when bottomhole pressure is being reduced by the downhole pump to its minimal value, specified by the pump location inside the wellbore.
In case this happens the well is switching to Pressure Control until the formation pressure and allow pump produce efficiently above minimal bottomhole pressure value.
Despite that many wells are operating under Pressure Control de facto they can be still simulated as under Liquid control assuming that their liquid rates have been manually pre-set adequately by Production technologist.
See also
Petroleum Industry / Upstream / Production / Subsurface Production Operations / Well Control
[ Pressure Control ]
[ Liquid Control (Reservoir Flow Modelling) ]