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 sshdand
# systemctl start sshd.serviceare equivalent, as are
# systemctl isolate defaultand
# systemctl isolate default.target