It therefore does not necessarily require the use of radar to provide air traffic control using procedural separation minima. In procedural control, any period during which two aircraft are not vertically separated is said to be "level change". In some cases, procedural separation minima are provided for use with radar assistance, however it is important not to get this mixed up with radar separation as in the former case the radar need not necessarily be certified for use for radar separation purposes, the separation is still procedural.