What's the meaning of DOUBLE and SINGLE for the RPT-MOD setting of Retevis RT23

Retevis RT23 is a cross-band repeater HAM Radio and supports DOUBLE and SINGLE repeater modes. Do you know what's the difference between the two modes?

SINGLE repeater mode

If you set the repeater mode to SINGLE, your Retevis RT23 only can receive the one repeater that you set.

such as if you set the RX=439.325MHZ, TX=146.325MHZ, you can receive the repeater whose RX is 146.325MHZ, and TX is 439.325MHZ.

DOUBLE repeater mode

If you set the repeater mode to DOUBLE, you can receive the two repeaters and, you only need to set a channel.

such as if you set the RX=439.325MHZ, TX=146.325MHZ, you not only can receive the repeater whose RX is 146.325MHZ, and TX is 439.325MHZ, but also the whose TX is 146.325MHZ, and RX is 439.325MHZ