Dimension Look-up during Transformation

Give this a try with Conditional BR

1. Create a conditional business rule and do something like this

Dim account As Integer = args.GetSource("Ac#").XFConvertToInt
Dim ud2 As String = args.GetSource("U2#")
								
If account >= 1000 And account <= 2000 Then
 If ud2 = "valid" Then 'check for valid U2#
  Return ud2
 Else
  Return "None"
 End If
Else
 Return ud2
End If

2. Attach this BR to your UD2 Transformation

Just to add to the answer already posted, if you need to lookup whether the incoming UD2 string is a valid member in the target UD2 dimension, please avoid using BRAPI functions (like BRAPI.Finance.Members.GetMember ) in BRs like this since each call to a BRAPI function creates and destroys a database connection object from the DB connection pool, which is quite inefficient.  You can make this more efficient by storing a list of valid members in memory , and caching it in BRGlobals, so it doesn't have to use BRAPI lookup functions every time.  Below example for illustration purposes...