Some niche biohacking oriented companies make red light therapy lamps emitting a spectrum of 639, 660, 850 & 880 nm light citing a particular authoritative book on the scientifically demonstrated unique therapeutic benefits of each of these frequencies.
This isn't something we can comment on unless you actually provide the details of the particular book, the claims made, or the studies cited. Generally speaking though, infrared light therapy seems to be pretty useless in terms of muscular-skeletal effects.
Beurer on the other hand makes a lamp emitting 1250nm (IR-A and IR-B) wavelengths.
What do IRA & IRB refer to, and what might be the rationale of designing a lamp to this wavelength?
IR-A refers to the near-infrared range, around 750-1400nm. IR-B refers to short wavelength IR, around 1400-3000nm.
Beurer's device uses a halogen bulb rather than LEDs, so it emits a range of wavelengths, rather than the single wavelength of an LED. 1250nm is merely the wavelength of peak intensity, and is in the IR-A range, but the device will also emit some visible and some IR-B.
As Beurer market their device purely as a heating lamp and do not make any therapeutic claims, the most likely explanation for their choice of peak intensity wavelength is that the bulb chosen provides some light in the visible spectrum, so that the user can see when the device is operating, but not so much light that the device would be uncomfortable to look at, as would be the case with a shorter peak wavelength and the greater exposure to visible light that would come with that.