The source text explains it fully: this is an RF gain control that operates on the DRIVER stage of the Tx — i.e. not on the final PA stage.

Once we've established that, I hope someone else may be able to improve on my actual wording.

Note that the EN might have been better expressed as Driver Gain Control — which has also often been referred to as simply 'drive control' — seems almost as if the writer couldn't make up their mind which to use!

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Note added at 1 jour13 minutes (2015-09-12 16:42:40 GMT) Post-grading
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Merci, Antoine !
Do note that HERE, in this specific context, 'étage d'adaptation d'impédance' would NOT be correct for 'driver stage' — that only really applies where signal levels are generally of the same order of magnitude, as in for example a 'line driver stage'; the situation here with an RF drive + PA is rather different since the intended action is more the amplification than the impedance matching aspect.