grammar - rather usage in this sentence

Sentence:

"The Function itself does not use this value; rather the value in this register is used by device drivers and operating systems."

Does it mean the value in this register is used by device drivers and operating systems? or it means the value in this register didn't be used by device driver and operating systems.

Answer

Rather as used in the sentence

The Function itself does not use this value; rather the value in
this register is used by device drivers and operating systems

has been variously named.

For example, Quirk et al. in A Comprehensive Grammar Of The English Language (p635) list it under Conjunct > Contrastative.

And The Cambridge Grammar Of The English Language (CGEL, p778) calls it a Connective Adverb of Addition and comparison (likeness and contrast). Included in the CGEL's list is the near synonym to rather in this context: instead.

The word rather can be omitted from the sentence without changing the meaning. But its presence serves to emphasise the contrast. I suspect that rather is less common than instead and may be perceived as a little more formal:

The Function itself does not use this value; instead, the value in
this register is used by device drivers and operating systems.