the eurorack format is a very common system format when it comes to modular synths. there are tons of modules and manufacturers on the market and the pocket operator modular series is is indeed eurorack compatible. this means any POM system can be used together with any other module, from any other brand. while the pocket rack format is slightly smaller than eurorack, you need some minor tweaks to fit these modules directly into your eurorack system.
this article will help you out if you want to 'eurorackify' your pocket operator modulars: