The ultimate objective of piece parts selection is to improve the reliability of the space power supply. Parts applications review is the responsibility of the Component Engineer. One of the distinguishing features of space power supplies is the care with which piece parts are selected. The power supply design is reviewed in order to maximize the use of preferred space grade parts. Preferred parts lists may be MDI generated, customer generated or established by the government (such as NASA PPL-21).

Various preferred parts lists (PPLs) exist, many derived from MIL-STD-975. The advantage of using parts from these lists is that they represent mature parts with considerable heritage. Preferred parts lists solely based on heritage may lag the present day by twenty to thirty years. Many of the components may be commercially obsolete, leading to lower reliability. The equipment becomes heavier and larger, leading to other design compromises. The manufacturing processes to make the piece parts accumulate change over the years, invisible to the activities maintaining the preferred parts list.

The motivation for these types of designs is not only using parts on the PPLs, but also the monetary costs for minimum lot costs of the parts.

Very often, large excess amounts of space qualified piece parts have been purchased on earlier contracts, and the designers are mandated to use these parts until stock on hand is depleted. The resulting designs may be non-optimal.