VOSA helps you to build and/or improve the observed Spectral Energy Distribution (SED) for the objects in your file in different ways.
First, you can upload your own photometry into VOSA for each object including it in your input file.
If you include your data as magnitudes or Jy, VOSA will transform them into erg/cm2/s/A using the information for each filter provided by the SVO Filter Profile Service.
You can search in VO catalogs to find more photometry for your objects and those new points (if any) will be included in your objects SED. Again, if the catalogs provide data as magnitudes or Jy, VOSA will transform them into erg/cm2/s/A using the information for each filter provided by the SVO Filter Profile Service.
In the case that, for an object, there are several photometry values corresponding to the same filter but coming from different sources (user and VO, different VO catalogs, same source at different epochs...) VOSA will average them and include the average value in the final SED.
Every observed SED will be dereddened using the value for Av provided by you in your input file or in the "Objects:Extinction" tab (with the option of searching VO catalogs for extinction properties).
For each object, VOSA will try to detect the presence of infrared excess using an automatic algorithm.
Then you have the option to inspect (and optionally edit) the final SED object by object.