Lone and remote workers
The arrangement that most often has not been thought through.
Somebody working alone cannot be reached by a first aider on site because there is no site. The arrangements that apply are different in kind: check-in systems, a means of raising an alarm that works where they are, and a plan for how help would physically reach them.
The common failure is a check-in arrangement that depends on somebody noticing an absence. A system where nobody is monitoring, or where the monitoring is a colleague who may be in a meeting, is not a system.
Communication coverage is the practical constraint. A worker in a basement, a remote area or a shielded structure may have no signal, and an arrangement built on a mobile phone has not accounted for it.
For genuinely remote work the response time may be measured in hours, which changes what training and equipment are appropriate. That is a different assessment from a workplace where an ambulance is fifteen minutes away.
Obligations regarding lone and remote work exist in most work health and safety frameworks and are specific. They should be worked from the applicable regulations and codes rather than from general first aid guidance.
It is worth adding that lone worker arrangements should be tested rather than documented. Having somebody actually trigger the alarm from the location in question, and timing what happens, is the only way to find out that the signal does not reach or that the monitored number rings in an empty office.