Tested in RiverWare 6.7 development:
  Open Data Object context menu: Create Similar Slot for Different Objects...

This option is enabled for only Time Aggregation Series Slots. This "Create Similar" feature creates additional Time Aggregation Series Slots on the same data object. But those additional instances are made to refer to (aggregate) the analogous slot on different simulation objects. The following sequence of images illustrates this.

Importantly, this context menu operation does NOT create similar slot instances ON other data objects.