Each analog input on the Quantum-series interface features a combo jack. This convenient connector accepts either a ¼-inch phone plug or an XLR plug. Mic Inputs 3-8/4 are located on the rear of the unit. Each input on your Quantum-series interface is also auto-sensing. When an XLR is connected, the signal is routed to the preamp. When a TRS is connected, the preamp is bypassed and the input signal is sent directly to the ADC. When nothing is connected, Channels 1 and 2 on Quantum will default to line inputs, Channel 3-8 will default to mic. All analog inputs on the Quantum 2 default to mic.
You can read more about this on page 3 of the manual: https://pae-web.presonusmusic.com/downloads/products/pdf/Quantum-series_Owners_Manual_EN-061020172017.pdf