Lockout/Tagout (LOTO) is regulated by OSHA 29 CFR 1910.147, and it exists because people die from unexpected energization. The six steps: (1) Notify affected employees. (2) Identify all energy sources - electrical, pneumatic, hydraulic, mechanical, gravity. (3) Shut down the equipment. (4) Isolate the energy source (open the disconnect, close the valve). (5) Apply your lock and tag to the energy isolation point. (6) Release/restrain stored energy (bleed air lines, discharge capacitors). Only then verify de-energization with your meter. In that order. Every time.
Multiple energy sources are where LOTO gets complicated. A machine with a 460V three-phase motor, a 120V control panel, a pneumatic cylinder, and a hydraulic clamping system has at least four energy sources that each need isolation. Each gets its own isolation point; your lock goes on each one.
If multiple contractors are working on the same equipment, each worker puts their own lock on the same device - the machine cannot be re-energized until every worker’s lock is removed. OSHA 1910.147(f)(3). Never share a lock or let someone else apply your lock.