man systemctl
:
Parameter Syntax Unit commands listed above take either a single unit name (designated as NAME), or multiple unit specifications (designated as PATTERN...). In the first case, the unit name with or without a suffix must be given. If the suffix is not specified (unit name is "abbreviated"), systemctl will append a suitable suffix, ".service" by default, and a type-specific suffix in case of commands which operate only on specific unit types. For example,
# systemctl start sshd
and
# systemctl start sshd.service
are equivalent, as are
# systemctl isolate default
and
# systemctl isolate default.target