What does "300K" mean in terms of image size/resolution?

I've been applying for jobs on an online board, and I keep seeing European clients saying they want their images to be delivered in 300K. For example, the one I'm looking at now says the freelancer has to deliver images in 15 x 15 cm, 300K. Can anyone tell me what the 300K means?

At first I assumed it means 300 DPI, but that's an imperial measure, so perhaps it's pixels/centimeter? There's a drastic difference in resolution between the two (1772px vs 4500px). I've seen this unit of measure in a number of places, so I guess I'm supposed to know it, but no amount of Googling has turned up an answer.

Appreciate any help! 🙌