1st Switch Option: Mic Attenuation

As usual for every Sony recording portable made (exception: TCD-D10 PRO model), there are just two Mic input Sensitivity settings.  Although how this function is labeled changes from the previous two DAT models, it does exactly the same thing on all 'pro-consumer' type portables produced since the early 80's.   Where this function was previously named (L) and (H) for Low and High MIC (preamplifier) SENSitivity, the D100/M1 models now revert to nomenclature not seen since the oldest 1^st generation TCD-D3 consumer portable DAT and (analog cassette) WM-D6C models.

The microphone preamplifier will clip at different Mic input signal levels depending on the setting of this switch.  With a Mic switch setting of 0 dB, about 8 millivolts RMS (-30 dBv) is the maximum (manual level setting without using the limiter).  However, in the 20 dB setting, this is a more generous (read practical) 150 millivolts RMS (-4 dBv).
The 0 dB (boost mode) setting is also a much lower mic pre quality mode with much higher (high frequency) distortion that loses the full bandwidth clarity available in the more conservatively configured 20 dB ATT setting.  Low frequency bandwidth is reduced when boosting the microphone preamplifier's gain.  Preamplifier bass is excellent when the Mic setting is non-boosted (20 dB ATT) showing only 3 dB down at 7 cycles. In contrast, it's bass response is mediocre when set at the 0 dB ATT boost mode with the 3 dB roll off starting much higher at 20 cycles and 1 dB reduced at 50 cycles.

VERY IMPORTANT TIP:
Consider the 20 dB ATT or (L) setting as the NORMAL microphone input setting for most all microphone signals with Sony brand recorders.  Only go to the boost mode setting of 0 dB or (L) when not having enough gain with a Level knob setting up full at #10 to reach a VU signal peak of  -12 dB below 0 dB VU.  Otherwise, you will clip distort or overload the microphone preamplifier if using the 'boosted gain setting' under any other conditions. 
This is consistently opposite to what is suggested in all the Sony user manuals that claim the 0 dB or (H) 'gain boosted' settings as the normal setting.  This has resulted in untold multitudes of their customers to make totally useless and distorted recordings even if using Sony brand microphones 'recommended' as ideal for these exact models!
2'd Switch Option: MIC or LINE
Because the same jack serves for both Microphone or Line inputs, check to insure this setting is correct.  The voltage gain difference between Line and 20 dB MIC input setting is about 17 dBv less gain when in the Line position.  In addition, it's almost impossible to overload these new decks with a very hot signal when using the Line input.  So for running 12- 48 volt (phantom) powered or large diaphragm microphones and for mixing board feeds, using the Line input is the natural 1^st choice if not the ONLY choice.  Just be sure to check that this switch is in the proper position.
3rd Switch Option: MANUAL, MIC LIMITER, or AUTO(AGC)
Additional new features include a microphone LIMITER and a newly designed AUTO(AGC) as two of the 'MANUAL, MIC LIMITER, AUTO (AGC) recording level controlling switch options.  The LIMITER/AUTO(AGC) options seem to work only when using the MIC input, being disabled for LINE inputs.  However, using the MANUAL level knob for microphone recording will always produce the most consistent and satisfactory recordings; assuming the recordist sets proper VU levels of sufficient headroom for handling 'unexpected'