Like the cube viewer, the Arc subset editor is built to generate MDX set expressions. Most of the functionality should be familiar to what you have experienced in Architect or Perspectives: all elements, filtering, ordering, etc.
Create a new subset
The subsets are per hierarchies, the default hierarchy name is the dimension name. To create a new subset, from the menu list, open the dimension, then the hierarchy and click the [New Subset] button:
Show only the first 10,000 elements
The subset editor will show by default only the first 10,000 elements but if you filter or search, it is going to filter or search on all elements of the dimension.
MDX record expression
There is no Record Expression option as the Arc subset editor is always recording the MDX expression. You can edit the MDX expression manually.
The Arc subset editor tests the MDX expressions by executing a query against the }ElementAttributes cube of the dimension. This means that any dimension that you want to use the subset editor against must have at least one attribute. Arc will prompt you to create one if it doesn’t exist.