That is the build number. The same is done for betas, Android (mobile) beta release notes. There is also an internal build number that is used by the store for upgrade purposes.
The first 3 numbers are the version, in this case 8.6.0. Version numbers for public releases should be the same. The build number is just done for tracking purposes by the devs.
Yes, it is possible that if you are jumping back and forth between different appstores, you could run into a conflict with the internal build numbers so you may not be able to upgrade from 1 to the other, but there really isn’t a reason a user should be doing this.
