Unfortunately, there is no standard.
Everybody does it differently.
For many years, it was slides.
Most often 20, but sometimes 10, or even 5.
So right behind me where I sit at the computer are over a dozen notebooks, each filled with hundreds of dupes of my best slides.
Hopefully those will become obsolete- they are expensive, fussy, need tiny little labels with 4 lines of microscopic type, and have a limited lifespan.
More and more, calls for public art are accepting digital images, and some are requiring them.
What I have started to do is shoot many fewer slides, and in many cases, no slides at all. The digital images are so much easier to deal with, and online, for around a buck, I can email my image and get a slide in the mail a few days later.
So I shoot almost all digital, and still have slides made of one or two of my best images of each work, for backup.
By the way, have you signed up for CAFE yet?
Its easy, its free, you upload your images once, and then can enter a call in just a few minutes of work.
They charge the agencies to list, so only a few are doing it, not all opportunities out there are listed, but I think its the wave of the future- it used to take me a half a day to prepare a proposal, and then, as I got good at it, an hour- but here, online- 5 minutes.