By default Suunto EON Steel has four different dive modes: Air/Nitrox, Trimix, CCR, 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.
Air/Nitrox mode is by default in graphical style and all the others in classical visual style. You can change this and other settings, as well as create additional dive modes using Suunto DM5. See Customize dive modes with DM5.
Air/Nitrox, Trimix and CCR modes are intended for diving with decompression information, like no decompression stop time or decompression ceiling/ascent time.
By default the decompression algorithm used in Suunto EON Steel 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 with Air/Nitrox, Trimix or CCR mode, there is no decompression calculation available and ERROR is shown in decompression information fields.