Holding a scenario means to temporarily prevent your GNSS receiver from continuing to move along its scenario trajectory (i.e., halting the trajectory), while the simulation continues to run otherwise (time continues to elapse). This can be done manually, by pressing the [.] / hold key, or by using the SCPI command SCENario:CONTrol, see SOURce:SCENario:CONTrol.
The display will show the HOLD icon in the upper right corner.
Note: Holding a scenario is not the same as arming a scenario (see Scenario Start Variations).
A typical use case for holding a scenario would be to simulate a red traffic light.