Essential requirements to sing without fear to damage vocal folds are:
1 - technique control
2 - consciousness and sensitiveness about personal limits
I think that we must remove some doubts:
- vocal range = all frequencies and noise between our extreme vocal limits (shrieks, yells, several noises, hissing sounds)
- vocal extension = all frequencies emitted with cleaning, with well controlled formants, without excessive effort
"Range" interval is larger than "extension" interval, that is more comfortable and safe.
Now we must distinguish between EFFICIENCY and EFFICACY.
An efficient voice involves a complete physiologic control of our vocal system, to obtain properly our vocal objective.
A voice is effective, not efficient, when our vocal system wants to obtain a good result in any case, at any cost.
When we can't obtain an efficient voice we move toward an effective voice with consequent incorrect behaviours of, and damages to, our vocal system.