The low point drain is used for draining fluid from the piping system, such as for repairs or prior to inerting for layup. The high point vent is used to aid in draining or to bleed air out of the system when filling. I take it that piping systems are not your area of expertise? You might want to purchase a Mark's handbook for mechanical engineers or attend an educational seminar if you would like to be more familiar with piping system terminology or design.