Detail View is shown when you click on View Details option from right corner ellipses icon Actions-Icon of any Item widget on the board. It opens the Item in new browser tab. Related sections(Steps to Reproduce, Attachments, Comments etc.) are shown under Description section.

Detail-View-In-Epic

Modal View is shown when you click on Item ID of any Item widget on the board. A popup opens up and Related sections(Steps to Reproduce, Attachments, Comments etc.) are shown in form of horizontal tabs next to “Details” (Description) section.

Modal-View-In-Epic