Switching means making an in-country application to change immigration categories.
Switching to PBS dependant
Unless one is currently in the UK with leave as specified in 319C(h)
, one cannot apply to switch
to PBS dependant leave. In such case, one must apply for entry clearance
from outside the UK
In simple words - if you are not in the UK as a PBS dependant then you cannot switch and need to apply for entry clearance from outside the UK.
An extension application is a leave to remain application to "extend" stay in the UK under the same immigration category.
One cannot apply for leave to remain (extension) from outside the UK. One can only apply for leave to enter (entry clearance) from outside the UK.