It could be anything, I used to have a sign that said " Clean Boiler every 100 working hours.". That was engraved on a brass plate attached to said boiler
B.E.
You are judged not by what you know, but by what you can do.
The DOE Federal Energy Management Program published Operations & Maintenance Best Practices Release 3.0 which has a boiler checklist with maintenance frequency. Very useful and can be downloaded from the link below.