Meaning of "catch you on the flip side"

I received an email from a coworker, and we're not that friendly. Actually, we're not friends at all, just two good colleagues. In his email, he wrote as his last sentence

Catch you on the flip side!

What does this expression mean?

Note that I was supposed to get a job, and the answer was no. The email was kind of sad.

The phrase "catch you on the flip side" is a very colloquial way of saying "See you tomorrow". It's unusual and unprofessional to see it in a business email.

I guess it depends on the context.

Yes, it was derived from the "flip side" of vinyl records, however it became a phrase equivalent to: "see you later/soon". But again, to me it depends on the context. For example, if someone is about to do something life threatening (or over exaggerated as life threatening) it could mean "see you in the after life", making the flip side, in this case, heaven. For example:

Oh my, I'm gonna jump out of a plane now… Catch you on the flip side.

Another instance where it could be used is obviously just "see you tomorrow". I once heard a theory that the flip side meant something or other about the Earth rotating through its 24 hour cycle, hence making the the flip side the following day.

Finally, the way I always thought it was taken is when you are saying goodbye to someone who you are unsure whether you may see again. Such as… Well tomorrow is my last day of high school so quite relevantly saying goodbye to classmates and teachers you may never run into again. Or perhaps someone you befriended one day on city transit and probably won't run into again. Or maybe a coworker leaving… Whatever it may be, I thought the flip side was just symbolic for the unknown. To sum it all up, I guess to me it meant "It's uncertain we'll meet again, but I hope we do."

I believe it means something like "See you next time we meet", often with an implication that the next time is known, assumed or hoped.

Here is a good example of where the next time is hoped: (Yes, I'm citing the Urban Dictionary, look at number 4)

In the early stages of spaceflight, NASA developed a technique that saved much needed fuel to return space craft back to Earth by using the moon’s orbit like a slingshot. Astronauts in space and ground control would lose radio signal once the space craft makes its way to the “back-side” of the moon and away from line of sight with Earth. They would of course regain radio signal when they reach the other side of the moon (the “flip side”). It was customary for Ground Control to say “Catch you on the flip side” before they would lose radio signal. The none-space working-class community adapted the phrase as a “good-bye” or “see you later.”