In this case, it refers to the literal letters written in the signature of a letter, which presumably indicate that whoever wrote it studied a bunch to get into school, paid a bunch to go to school, graduated with that degree, and should be respected, because they may have the knowledge, willingness, and resources to take the words in the letter further, if they want to, given that they have already somewhat attached themselves to the cause in the form of the letter.
[0] https://law.stanford.edu/office-of-student-affairs/the-docto...
Jonathan Coulton released a cover of "Baby Got Back" with a melody he composed himself. The entire song, melody and lyrics, was appropriated without permission by Fox and performed on an episode of Glee by the cast. (I say "lyrics" because even the change of "Mix-a-Lot's in trouble" to "Jonny C's in trouble", referring to Coulton himself, was preserved in the Glee version.) When Coulton wrote a letter demanding at least acknowledgement of the original elements he contributed to the cover song, Fox insisted that their asses were covered because they paid the ASCAP fees and did not have to seek permission for, nor acknowledge, the use of a cover of a song they licensed.
If you understand laws as weapons with which the rich protect themselves from the poor, a whole lot of shit starts making sense...