If I am drawing in 3D and I want to draw a line with a specific angle, how do I choose the plane that it is drawn in? Cos whenever I specify an angle the line is drawn in the horizontal plane
1) If you are working in the primary orthogonal plans, turn on ucsortho and then use the view command or the naavcube to make whatever view you want current. The ucs will automatically rotate to that plane. You still might need to set the origin before working. Saving UCS's helps.
2)If you have solids, then UCS face works well.
3)DUCS mode can help.
4)The suggestion about the selectable UCSICON works too.