WindowEquivalentLayer - SHGC vs Solar Transmittance at Normal Incidence

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"Center-of-glass" means that it accounts for only the heat flow from outside to inside through each layer of the window assembly, and does not account for any heat flow through the frame and other edge effects. You can read more background on the concept here.

The SHGC values you see reported are for the combination of all fenestration layers (glazing, gas, shades, etc.). The SHGC value of your window assembly without curtains (0.556) is the ratio of how much heat enters the building through a window divided by the amount of incident solar radiation that hits that same window. This falls in line with typical values for double-pane assemblies.

For future reference, if you can't determine from the Input-Output Reference documentation how EnergyPlus uses input parameters for specific calculations, you want to review the Engineering Reference documentation. Here is the section for the equivalent layer fenestration model that you are using.