Kupas Rabeinu, “Our Rebbe's Fund,” is a charity of Machne Israel. The Rebbe would earmark the monies donated to this magbis to general purposes, via the general Machne Israel account. The following is a fascinating short audio clip of the Rebbe speaking about the virtue of donating to this fund. It is from the Purim farbrengen of 1969 (Courtesy JEM-Ashreinu).
The Rebbe would traditionally make the appeal for this fund at the festive Purim farbrengen.