In proportional control, a controller’s algorithm can adjust the correcting variable anywhere between 0 and 100%, so that it is exactly as required to keep the process stable and on setpoint.
If the control type is dual, both primary & secondary outputs are available, each of which can give proportional control. When the proportional band(s) are correctly tuned, the process variable is maintained at a steady value, avoiding the oscillation characteristic of on-off control.
Proportional control is most commonly used in conjunction with integral and derivative action to give PI. PD or PID control. If integral action isn’t used, the biasing of the proportional bands may need resetting periodically to remove any control deviation.