What happens when you select two (2) or more columns as keys in a table ?

[FTong]

I understand that when select two (2) columns -or more- as key in a table app sheet creates a virtual column that combines the columns selected as keys.   

However, what I need help understanding is how AppSheet treats/handles this new "combined" key when I REF this table from another table (that only have one of the key columns in it)?

1) Do I need to have a matching "combined" key?

2) Do I only need to have one of the values in the combined key to match?