You can include multiple different parts in one drawing file (one order) for 2D designs if the thickness of each part is the same. For example, you could design a sheet metal box consisting of three separate 1/8" thick pieces in one drawing file (one order). To include multiple designs simply include each part in the drawing on the same screen. If you receive an error about multiple parts it is usually because your parts do not conform to the 2D part definition. You can also have differing quantities. For example, you could order 10 of one part and 20 of another. To order 10 of one part and 20 of another include one copy of the first part, two of the second and then order 10 sets.
If you have multiple thicknesses or different materials you need to create separate drawing files and place separate orders since each order is cut from one sheet. (You could use one drawing file, set a thickness, place an order, change the thickness and place another order but it is recommended to make separate files for separate orders for record keeping.)
3D designs permit only one design per drawing file (one per order). Files that split a single 3D design in two using the Air Inside feature are not accepted.