The datastream feature in PosApp software is only available to users with the SimGEN mode of PosApp.
The datastream output only provides data for satellite signals that have been allocated to a simulator channel, i.e. are being simulated. Channel allocation is determined by various scenario conditions / settings (of particular interest, see FAQ11667). The general rule, under 'normal' scenario conditions, is for a satellite signal to be allocated to a simulator channel as long as it is in view of the vehicle's antenna. But there are of course many other conditions which might lead to a satellite not being simulated, e.g.
You must of course have enabled output of 'Signal' data via the datastream. This is done via the scenario tree [Options-->Data streaming definition file-->Vehicle-->Antenna]. You will only receive signal data for those signal types which match your Vehicle/Antenna selections in the scenario tree [Vehicle-->Antenna-->Signal types]; which in turn must match the capability of your simulator equipment.