Star Trek: 10 Starfleet Rules You Never Knew Existed

It was a lot easier to become a Starfleet officer if you were a member of a Federation-aligned species, but the opportunity was open to anyone seeking to join.

The difference is that folks from non-Federation species would have to get a letter of recommendation from a command-level officer before they would be allowed to apply for the Academy. Nog, for example, was able to get a recommendation from Sisko in the DS9 episode Heart Of Stone (after convincing him that his desire to serve was genuine).

Heart Of Stone was the only time that this requirement was mentioned, but it's reasonable to assume that all other non-Federation alien officers like Worf and Saru also had to get recommendation from a command-level officer before applying. Starfleet typically doesn't want to exclude people just because of where they come from, but this rule does give an advantage to aliens who are Federation members, allowing them to simply jump straight into the entrance exams before having any real-world experience with Starfleet.