The PI Module Database (ModuleDB) is a hierarchical data structure introduced by OSIsoft years ago. This hierarchical data structure comes free with every PI server and is often overlooked (people don't bother configuring it).
You see, the purpose of the ModuleDB is to account for the units of your physical plant. Perhaps you are a biologics manufacturing facility with bioreactors, mixing tanks, centrifuges and chromatography columns. Or perhaps you're a sulfuric acid plant with a blower, furnace and stack. You have equipment big and small whose I/O are sending data to the PLC/DCS that then send the data to PI. You can describe your physical units with the PI ModuleDB.
The big deal with the ModuleDB is that you get to associate these I/O (tags) with the unit to which they belong; and then you get to label (create an alias) that tag something other than the instrument address (which is gibberish to most people anyway). For example:
is not as memorable as
T447 Optical Density
The reason having unit/aliases is important is that it makes PI relevant; it brings PI tags closer to the community of users that talk about it. Walk around your plant and listen to the operators, supes, engineers and managers talk. Are they talking about the
pH on Bay 1or are they talking about
Chances are, their words refer to some parameter/measurement on the unit rather than to the tagname... which is mostly known by just the folks in automation or instrumentation.
Configuring the PI ModuleDB to represent your physical plant and then associating the relevant tags to those units via alias is a high-bang, low-buck activity that will pay dividends for years to come.