The R1/C1 network makes a low pass filter when the wiper is at the grounded end of the tone pot, and there is a treble cut. The C1 cap bypasses R2 when the wiper is adjusted so that it is at the top end of the pot and it creates a treble boost.
The 100k output volume and the 100k tone pot are always in parallel as a constant load. It’s suggested to used a linear taper pot for the tone control and a log (audio) taper for the volume control. Suggested values for initial experimentation are R1=10k, R2=47k, and C1=0.022uF.
Some signal loss, as with any passive network is the limitation of this combined tone control. However, many guitar pedal designs have strong enough output signal level, and this tone control is an excellent option for those circuits with enough drive.