When you create a new dimension and select the field from field list, then it will be shown in the selections tool. If you type the field name in the expression editor then it will be treated as function, it gets the "=" sign before field name and it won't appear under App Dimensions on the selections tool.
thanks for your reply. I am aware that using field as a dimension will add it to the selections tool. However, in many cases I use variables as dimensions, which allows users to select which dimension they want to see. For example in the image below, users can choose whether to look at Granted Rights (on the right) by Category, Sub Category or Product by having the buttons change the value of a variable (vGrantedRight). That variable is then used as a dimension in the table.
However, this leaves the fields (Category, Sub Category and Product) out of the selections tool. My question was meant to ask: is there a way to add fields into the selection tool, other than using the field as a dimension directly? Having looked into this, it seems that for the time being that isn't possible.