px,py and pz prefixes are reserved for Pega, so you cannot create unique artifacts using any of these three prefixes.
In general terms, artifacts prefaced with py are Read/Write so can be overridden in your application. For example, you can use the .pyWorkPhone property in your application and present it on a UI screen for a user to update. You can also extend these artifacts by creating copies of them in your application.
In these same general terms, artifacts prefaced px and pz are Read Only - and cannot be overridden or extended by your application. The difference between these two artifacts (px and pz) is that artifacts prefaced with px are typically used to support your application. For example, when creating properties, you can specify the UI control type such as pxTextInput or pxDropdown. Artifacts prefaced with pzsupport (Pega) internal system processing.