In View Mode, a double-click on an Attribute Input or Output block has no effect. In the Edit Mode, when you drag and drop an attribute block from the palette, configuration wizards launch to help you configure the attribute.
Attributes support a maximum of 24 input or 24 output blocks of Numeric, Enum, or Bool. Attributes expose values at different stages of the Metasys system for a user to see at a glance or connect to/from.
See Editing Blocks in Edit Mode for information on editing Attribute blocks.