It is a nice security measure; it means that no other service can accidentally read or write to the same files. An older form of the security measure was to cre

It is a nice security measure; it means that no other service can accidentally read or write to the same files. An older form of the security measure was to create new system users like “ftp” and “mail” so that the ftp daemon couldn’t be tricked into overwriting someone’s mail queue and so on. The downside is that this takes a certain amount of effort on the part of the system administrators and distros to arrange; the bookkeeping becomes a significant source of manual effort. The DynamicUser= setting tells systemd to invent a new user on the spot, with an id that is not in use by any other service. You can add as many such services as you like¹ without having to do any extra work to keep /etc/passwd updated. ¹ as many as you like as long is it isn’t more than several thousand, unless you recompile systemd with larger limits.