This may be a
UK .vs. US language difference, but to me :
A 'filament' is a thin wire, heated electircally. A heater (in a valve)
is a particular type of filament
A filament that's also the cathode (as here) is a 'directly heated
cathode'
A filament that heats a separate cathode is called a 'heater'. The result
is caleld an 'indirectly heated cathode'
This is the way I learned it, and I'm in the US.
My own opinion on the subject has been colored by "Star Trek" and
"quantum filaments".
Peace... Sridhar