Two tables(years), same structure, one relation, what function is used in this case?

Hi,

It was quite fun to solve your Problem. 

At First two sample Tables based on your Information.

The Data Inside of the Tables.

Then I build up a new calculated Table I call it Clients. With all Unique Clients as the first Column with this Dax Code.

Clients = DISTINCT(UNION(ALLSELECTED('Client-Year2018'[Client]);ALLSELECTED('Client-Year2019'[Client])))

After that you can calculate for each unique Client if it present or not with the Dax Code below in a new Calculated column.

I Modified it a bit to know if a customer comes new in 2019.

Classification = 

var present2018 = LOOKUPVALUE('Client-Year2018'[Client]; 'Client-Year2018'[Client]; Clients[Client])
var present2019 = LOOKUPVALUE('Client-Year2019'[Client]; 'Client-Year2019'[Client]; Clients[Client])
Return
IF(present2018 = Clients[Client] && present2019 = Clients[Client]; "continuators"; IF(present2018 = Clients[Client] && NOT(present2019 = Clients[Client]); "Lost"; IF(NOT(present2018 = Clients[Client]) && present2019 = Clients[Client]; "New")))

The new Table looks like this.

I hope this will help.

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