Phase II - This phase specifies the definitions, guidances, and appraisals given in Section 1.1 for those instances where ARINC 600 LRUs are of a design catering to mechanical, electrical, and environmental control that is substantially beyond those features for Phase I. Phase II, therefore, is another step in the evolutionary development of avionic installations. It is intended to enhance the operator's satisfaction with avionics built to and installed in accordance with Phase II specifications. This satisfaction is meant to encompass life cycle cost, reliability, maintainability, etc. As with Phase I and ARINC 404/404A, it is likely that aircraft manufacturers will find it necessary (convenient) to sometimes mix Phase II LRUs and Phase I LRUs in the new rack. When this is done, the installation criteria for Phase I will prevail. However, it will not be proper to install Phase II LRUs in ARINC 404/404A shelves since these shelves will not be able to meet certain Phase II requirements. The standard rule is, as stated above, always install a mix of LRU phases in the lower-numbered phase shelf/cabinet and no more than two phases may be mixed on one shelf. Put differently, a Phase N + 1 LRU can be used in (and only in) a Phase N or a Phase N+ 1 Rack/Cabinet. Similarly, a Phase N LRU can be used only in a Phase N or Phase N - 1 Rack/Cabinet.