The selections blocks have a "StepStatus" property that is set to "Required" by default. That means that when the dialog is created and the selection blocks are empty, you see a red asterisk in the block and the OK/Apply are greyed out. When objects are selected in the block, the red asterisk changes to a green check. Once all selection blocks have green checks, then OK/Apply is enabled.
You change the StepStatus property to "Optional" or "Satisfied" so that a particular block does not require a selection. See the StepStatus documentation for CurveCollector or SelectFeature for more information.