Explanation/Reference:
Dynamic layouts allow nesting, and this is a very powerful feature in structuring your sections. The reason for this is that a section with multiple fields typically demands organization into groups that are intuitive to the end-user.
Nesting permits you to add a dynamic layout inside another dynamic layout. In the example below, a default dynamic layout contains three nested inline-grid layouts, which group fields with different kinds of information (such as First Name and Last Name in the third one). As the layout changes responsively according to screen width-from desktop to tablet to smart phone-the nested layout preserves the logical grouping of fields as the layout shrinks and then stacks to accommodate the space.

References: https://pdn.pega.com/using-layouts-structure-content-sections