Reference tables should also be specified at this point, but is optional. Reference tables will be included in the subset in its entirety and will not proceed to grab additional data as part of the subset. For example, you could have a table that represents states in the US - one row per state. When coming across the states table, setting it as a reference table will ensure all 50 states are pulled. Otherwise, if you were to say subset on users in CA, it would only pull the state of California. See 5 in Figure 1.