Drawing in Multiframe will also automatically align to joints and members in your model. 

You can control which parts of the model the cursor will snap to when drawing via the Drawing Settings dialog from the Geometry menu.  This dialog gives you options to snap to

·                      Joints

·                      Members

·                      Member quarter points

·                      A customised number of points along members

·                      Perpendicular point on member

The option to snap to a perpendicular point on a member is only used to snap the second end of a line been drawn such that the line is drawn perpendicular to the existing member.

When drawing in a 3D view the drawing defaults to be located in the current working plane. This plane is the plane oriented most perpendicular to the current viewing direction which passes through the current drawing depth.  The axes indicating the origin of the coordinate system highlight the axes that identify the orientation of the drawing plane.  When the cursor snaps to an existing item in the model the drawing will snap to this point in 3D space.

When drawing is a 2D view snapping to existing pasts of the model out of the current drawing plane can be a problem.  For example, when trying to draw a planar structure in front of an existing part of the model the cursor may snap to existing objects which can move the drawing from the current working plane.  To avoid this problem all drawing can be forced to occur in the current working plane by selecting the ‘Constrain drawing to working plane option’ in the Drawing Settings dialog.  When this option is selected, the cursor will still snap to existing parts of the model but drawing will be projected back to the current drawing depth.

When drawing or dragging you can stop the cursor snapping to joints, members and grid points by simply holding down the ALT key while performing the operation.