Why are some systemd services in the "masked" state?

When I run the command sudo systemctl list-unit-files (I think the sudo is optional), I get output which shows all services and their state.

Here is a snippet from my machine:

UNIT FILE                                  STATE
...
debian-fixup.service                       static  
debug-shell.service                        disabled
display-manager.service                    enabled 
dns-clean.service                          enabled 
dsmcad.service                             enabled 
emergency.service                          static  
failsafe-x.service                         static  
friendly-recovery.service                  masked  
fuse.service                               masked  
gdm.service                                masked  
getty-static.service                       static  
[email protected]                             enabled 
gpsd.service                               indirect
[email protected]                           static  
gpu-manager.service                        enabled 
halt-local.service                         static  
halt.service                               masked  
hostname.service                           masked
...

I wonder why some services are in the "masked" state. I think this means, "this is better than 'disabling', because the service cannot be started, neither by hand or by systemd".

How can I get more information about the state of a service unit?

Who has put the units into their respective state?

I tried, for example, sudo systemctl help dsmcad - that only brings up the documentation = ... line from the unit file. /etc/systemd/system/dsmcad.service

Note: Here I know exactly what dsmcad service is and what it does, I have installed it myself. I am more interested in a general solution.

mask is a stronger version of disable. Using disable all symlinks of the specified unit file are removed. If using mask the units will be linked to /dev/null. This will be displayed if you check e.g. by systemctl status halt.service. The advantage of mask is to prevent any kind of activation, even manual.

Caution: systemctl list-unit-files is listing the state of the unit files (static, enabled, disabled, masked, indirect) and has nothing to do with the state of the service. To have a look at the services use systemctl list-units.

hostname.service is masked as redundant because systemd sets the hostname (from /etc/hostname) very early on during startup.

This setting is provided by the Debian systemd package.

$ ls -l /lib/systemd/system/hostname.service
lrwxrwxrwx 1 root root 9 Apr  8 22:47 /lib/systemd/system/hostname.service -> /dev/null
$ dpkg-query --search /lib/systemd/system/hostname.service
systemd: /lib/systemd/system/hostname.service

Similarly, Debian can now run without a shell script to halt the system, it's handled by systemd-shutdown (source code here) instead.

If a service has been masked manually, the mask will be installed in /etc/systemd/system instead.

Services are also masked when they are removed on Debian/Ubuntu. I don't know why.