If you set the Polyline ID field to ***?*** you will be asked by RivEX if you want to create a new polyline ID field called RivID which will be a unique sequential number. You need to do this if the only unique number fields you have in your dataset are the ObjectID/FID fields as these will not be offered to you in the drop-down. ObjectID/FID fields are excluded from the Polyline ID list as some tools used by RivEX would not function correctly if these fields were the fields chosen to be the unique ID field.
Once the RivID field has been created you must use that as you polyline ID field.
If you have an existing unique ID field which is of type Long then you can select that instead of RivID.
If RivID already exists in your dataset then a numeric suffix will be added (e.g. RivID_1).
By setting the node fields to ***?*** you are asking RivEX to recalculate the node ID's. You would do this the first time you use the network with RivEX which ensures correct node numbers exist or after you have edited the topology of the network. See here for more information.