We are beginning siman 3. The Chayei Adam writes that if one recites mezonos on any food, they are yotzei bedieved. He explains that all foods fall under the category of mazon, sustenance, with the exception of water and salt, because all other foods provide nutrients to a person. The Gemara already teaches us that the bracha shehakol can exempt any food, but the idea that the bracha mezonos can exempt other foods that are not of the five grains is the chiddush of the Chayei Adam.
The chiddush of the Chayei Adam and the exception that mezonos does not cover water and salt are based on a mishnah in Eruvin. The Mishnah states that if one makes a neder prohibiting themselves from consuming mazon, they may not eat any food or drink but they are still permitted to consume water and salt. Chazal understand that even though they are necessary for life, water and salt do not function as mazon, because they have no nutrients in them.
The Mishnah Berurah in the Biur Halacha (167:10) concurs with the opinion of the Chayei Adam. He adds that the Elyah Rabba and Drisha also hold accordingly.
It is important to note that besides for the fact making mezonos on other items only works bedieved, when one has a safeik bracha, the correct action is to make a shehakol, not a mezonos.
If one recites mezonos on any food besides water and salt, they are yotzei bedieved.
Nonetheless, in a case of a safeik bracha, the correct bracha to make is shehakol, not mezonos.