On any given day there are hundreds, or maybe thousands, of assets driving a plant environment. Each application within the plant has various tasks hence establishing the need for assets of different sizes, cost, and criticality. All assets need to be cared for cost effectively; however, there are usually several assets per plant that are labeled critical assets which receive a little extra care. Review your plant's assets, identify and verify the condition of the critical assets regularly, and use the PM tools and methods that are available to optimize the integrity of those assets before other less critical assets. Based on the risk to business, on average less than 20% of assets within the plant should be labeled critical.

Source:  Maintenance and Reliability Best Practices by Ramesh Gulati