Do the psaserv and psacln groups have particular meanings?
These groups are created by Plesk, a common server administration panel that is generally used on web servers hosting websites.
The psaserv group is used for the server-side management scripts and also for parent-level directories that should not be shared between different websites.
I have not seen a psacli group, but on normal Plesk installations there is the alternative psacln. This group is used for client-side files, such as all of the files held within a website.