What is a cross signature?

I was told to get the cross signature done on the envelope containing my recomendation letter by the principal but I have no idea of what a cross sign is. How can it be done on the envelope?

A fairly common requirement, when letters of recommendation are handled by the person who is recommended, is that the recommender puts the letter in an envelope, seals the envelope, and signs his name across the place where the envelope is sealed. The idea is that the person who is recommended cannot then open the envelope and read the letter before sending it to the intended recipient. I would guess that "cross signature" is intended to mean this signature across the envelope's seal.

I haven't heard the expression "cross signature," but I know that some school admissions require the recommender to sign across the back flap of the envelope, so that it's clear it hasn't been opened by you to read the letter inside.