The iX3M / iX3M2 / iDive Color with paired transmitter during the dive, shows the current pressure in the tank (the "BAR" value) and the remaining bottom time ("RBT" value). Those are the main data that iX3M / iX3M2 / iDive Color shows.

You can set the reserve value ("RES") alarm. It means that the BAR data will become red if the BAR is below the reserve value or yellow in case the BAR value in below (reserve x2) value.

At page 25 of the Wireless Transmitter manual is available a picture that explain that function:
https://www.ratio-computers.com/support/support4/manual/EN_User_Manual_CC_Transmitter_2.1.pdf

As default it is a visual alarm only (not sound or vibration), but you can set the Reserve alarm ON in the ALERM SET menu (it means that the iX3M / iX3M2 / iDive Color will vibrate and make a sound when the BAR value goes below RES value).

Always remember that the wireless transmitter does not replace the pressure gauge.