Exterior elevations show each side of the building. Interior elevations show one wall of a room, with the items mounted on it and their heights.
Example: the interior elevation of a gym wall shows the wall pads, their height and where they stop at doors. The floor plan shows only a line.
Use elevations to confirm what the plan suggests. Count wall-mounted items there and read their mounting heights. Find the elevation markers on the floor plan first, because each one names the sheet and the view direction.