"We recognize and respect TSA’s authority to issue
security directives, however, we do not believe that TSA should use
them to make standing policy unless there is a compelling and
immediate national security risk that warrants them."
Source: Jeppesen President and CEO Mark Van Tine.
Concerning altitude/flight level, the term means to remain at
the altitude/flight level specified. Concerning other ATC
instructions, the term is used in its literal sense; e.g., maintain
VFR.
The Canadian Bushplane Heritage Centre in Sault Ste. Marie,
Ontario is dedicated to preserving the history of Bush Flying and
Forest Protection in Canada.