By default Suunto EON Core has two dive modes: Air/Nitrox and Gauge (bottom timer). Select the appropriate mode for your dive under Dive settings » Mode.
Air/Nitrox dive mode is a single gas mode. The Gas(es) menu under this mode will not allow you to add more than one gas. However, this can be changed through customization. See Customize dive modes with DM5.
The default dive modes us prominent style. You can change the style and other settings, as well as create additional dive modes using Suunto DM5. See Customize dive modes with DM5.
In DM5 you can also enable multigas mode for Trimix and other more advanced decompression dives.
The decompression algorithm used in Suunto EON Core is Suunto Fused™ RGBM. For more information about the algorithm, see Suunto Fused RGBM.
Gauge is a bottom timer mode and thus has no decompression information or calculation included.
After diving in gauge mode, decompression calculation is locked for 48 hours. If during this time you dive again, there is no decompression calculation available and Locked is shown in decompression information fields.