Hi Chris,
Chris wrote:what is it mean "shall be easily openable" ?
That is like asking how fast is fast ... when there is no definition of "fast".
The best answer, in relation to opening EERO (Emergency Escape and Rescue Opening) windows, would be "Can the window be opened easily by the average adult." If the window is hard to open, or difficult to open for whatever reason, then the answer would be that particular window does not meet the "shall be easily openable" requirement.
If the window requires extra effort to be opened, then an average adult 'might not be able to' open the window.
Yes, the window lock should not be higher than 44 inches above the floor to allow for access by children, however, the sizing of the opening and maximum sill height above the floor is based on the average adult being able to escape from inside, and from a fully equipped firefighter being able to affect a rescue from the outside.