Basic elements are sections, containers, rows, and columns. Since Pagex is fully based on Bootstrap, elements like containers, rows, and columns have some predefined style. For containers it is max. width, margins, and paddings, for rows negative margins and for columns there is predefined paddings.
Bootstrap is a mobile-first framework which makes Pagex also mobile first. That means all settings you are editing in elements will be added for all devices then if you need to specify a different setting for tablets you switch to tablet media query option and set different settings. Now mobiles are gonna have one option and tablets another option and all other viewports which are bigger that tablet are going to inherit tablet options.
Check Official Documentation for Basic Elements.