Ultra Low Noise mode is a special imaging mode (2020), which reduces read noise making it the lowest of any CMOS camera with comparable sensor. It is only available on certain camera models and may be greyed out on the ASCOM driver or your preferred capture software. Ultra Low Noise Mode is for scientific imaging, and is not considered necessary for amateur astronomy. The impacts of Ultra Low Noise Mode are slower image download speeds and a larger amount of USB traffic, which could potentially cause reliability problems when using a cheap PC with inadequate USB architecture. Seeing most astronomy capture PCs are small budget devices, we do not recommend using it for the above reasons, and because stacking subframes will reduce noise in any case.
Full Well Mode is a new feature (2022) available on some 16bit camera models such as the 26C and 26M series. It works by increasing the full well capacity, however this creates more read noise as a result and is not always beneficial compared to stacking multiple frames. In order to determine whether you need Full Well Mode, it would be best to perform a test imaging sequence under your skies with filters and scope and the examine the results in processing software, or perform a sensor analysis in SharpCap PRO. In general, it could be said that full well mode would be used with Altair Dual Band or Narrowband filters and long exposures, or without filters in a location with very low light pollution.