What should a UX Designer use to design responsively within the Salesforce Lightning Design System...

A grid system is a set of columns and rows that help to organize and align the layout and content of a web page. The Salesforce Lightning Design System (SLDS) provides a responsive grid system that adapts to different screen sizes and devices. The SLDS grid system is based on the CSS Flexbox layout module, which allows for flexible and dynamic positioning of elements. The SLDS grid system consists of the following components1:

Containers: These are the outermost elements that wrap the grid. They have a fixed width that changes according to the breakpoints defined by the SLDS. Breakpoints are the points at which the layout changes based on the screen size. The SLDS has four breakpoints: small, medium, large, and x-large2.

Grids: These are the direct children of the containers. They are the main elements that define the grid structure. They have a display property of flex, which enables the flexbox layout. They can also have modifiers that control the alignment, direction, and wrapping of the grid items3.

Grid items: These are the direct children of the grids. They are the elements that contain the actual content of the page. They have a flex property that determines how much space they take up in the grid. They can also have modifiers that control the order, offset, and visibility of the grid items4.

A UX Designer should use the SLDS grid system to design responsively within the SLDS, as it provides a consistent and flexible way to create layouts that work across different devices and screen sizes. The SLDS grid system also follows the SLDS design principles and best practices, such as clarity, efficiency, consistency, and beauty5.