To assign decking

Either:

select Layout > Slabs > Deck Assign

or
select the Deck Assign tool from the Slab Layout dynamic toolbar

The Deck Assignment Mode dialog opens.
Select a Slab Action of either One-Way or Two-Way. The Slab Action determines how forces are distributed from the slab to underlying members.

If One-Way is selected as the Slab Action, select the Deck Orientation to be assigned. If the deck orientation is skewed, enter the angle of the skew (between 0 and 180 degrees, where 0 is horizontal on the screen, measured counter-clockwise), or click on a Pick button next to Parallel To or Perpendicular To. If you click a Pick button, you will be given the opportunity to pick a beam or a wall, and the angle will be calculated such that the deck orientation is either parallel to or perpendicular to that beam or wall.

or
If Two-Way is selected as the slab action, the Drop Cap check box may optionally be checked to indicate that the decking is a Drop Cap. If the Drop Cap check box is checked, enter the Angle, then either select Square and enter the size, or select Rectangular and enter the offsets.
Select the Framing System (Composite, Noncomposite, or Concrete).
Note: Composite may only be selected if One-Way is selected as the Slab Action.
Select the deck property to be assigned by clicking the desired entry in the Deck Property list box.

Once the above information in the dialog has been entered, decking may be added using any of the following methods.