When you register a custom filter, the second parameter to PluginRegistrar.RegisterFilter specifies the filter type. In addition to 3D object filters, you can also have subcomponent filters (for points, edges, or polygons), property filters, and generic object filters.
Softimage uses the filter type to determine when and where a filter is available. For example, in the Select panel, subcomponent filters are not available when the current selection includes nulls or implicit objects. In an explorer view, only 3D object and object filters are available, and object filters are available only for certain scopes.
Note that for filters, the SIObject.Typeproperty returns siFilterFundamentalTypevalues, not siFilterTypevalues. For example, for a point filter, SIObject.Type returns the string SubComponentFilter, not the number 5 (the value of the siFilterSubComponentPoint constant).