Hi Jose,
From my understanding here’s the flow of data:

• List A, where you have both Client ID and Self Served
• There should be a unique link between Client ID and Self Served, where a Client ID should have one Self Served property
• List B, where you have Client ID and want to bring Self Served property, based on the information from Client ID of List B

Based on that, here’s what I would do

• Create a property with dimension format Self Served in the dimension Client ID, with a formula ‘List A’.’Self-Served’lBY LASTNONBLANK: ‘List A’.’Client ID’]. You can try this formula in the formula playground first
• Then you can just reference it in List B by calling ‘List B’.’Client ID’.’Self-Served’, calling the property ‘Self-Served’ you’ve just created in the dimension ‘Client ID’

Please note that I’ve used LASTNONBLANK because there may be several items in List A with the corresponding Client ID. Pigment needs a way to know which to take, so you can use LASTNONBLANK but also FIRSTNONBLANK or even FIRST or LAST. You can even also filter this by adding a FILTER like this ‘List A’.’Self-Served’EFILTER: ‘List A’.Period = ‘Current Period’]eBY LASTNONBLANK: ‘List A’.’Client ID’] or any other kind of FILTER that could be based on a defined set of properties that would reduce the number of relevant lines in the List A.