The term conventional describes a system composed of single subsystems (rectifier/battery charger, inverter, static bypass, etc.) with no inherent redundancy, also referred to as conventional stand-alone UPS. They provide basic power protection for critical loads or as a "building block" in parallel or dual bus systems.
As conventional systems have a relatively simple topology (compared to a modular or scalable system), they offer an excellent budget solution for small and medium data center applications. However, as they have a fixed rating - the availability of the protected load is strictly related to the UPS itself - they offer limited flexibility to adapt. Conventional UPS systems can be configured to operate in parallel (N+1) and/or dual-bus (N+N) applications to provide the ultimate system availability. In case one of the UPS fails, the other UPS system continues to run and protect the load. Please note that in parallel systems, each UPS should have the same power rating.