Ahmad: Your assessment is correct. The Measured-Index Multimode Fiber and Parabolic-Index Multimode Fiber components are expecting multi-mode spatially defined signals. If you use the Multimode Generator with all the power allocated to the primary mode then you will obtain a single mode laser signal with extra information on the mode field type and size (complex field data in the x-y plane) – this resembles most closely your laser in the lab (since it has a field diameter). This information is used by the Measured-Index Multimode Fiber and Parabolic-Index Multimode Fibers for the overlap integrals (used in the mode solver). The accuracy is thus pretty good. For the Linear Multimode Fiber block there is no need for spatial complex field information as we do not call up a mode solver (only a transfer function is applied).
If you connect a Spatial Receiver or Spatial PIN to the output of the Measured Index or Parabolic Fiber then it will apply the spatial aperture against the modal field envelopes. The resulting truncated field data will then be applied as an attenuation to the time-domain waveform of the signal before being converted to an electrical signal. So the temporal effects associated with the signal (multimode dispersion) are held by the primary signal (I recommend that you try out the View Signal Visualizer – this will show you the baseline signal (complex signal as a function of time) and also the complex field data in the x-y plane for each calculated mode (this data sets are separate)).