What is the source for the EMCONTRB variable in the IPUMS-CPS?

What is the source for the EMCONTRB variable in the IPUMS-CPS?

The IPUMS-CPS variable EMCONTRB is created from the CPS variable of the same name. You can access the source variable (EMCONTRB) in the March ASEC samples from 1992-present.

Hope this helps.