What does that mean ?
Part 4 is THE Reference Data Library (RDL) and as such describes all of the engineering terms / objects required to design / build & operate an engineering facility. As such ISO 15926:4 only describes the starting reference data. The posc-caesar RDL or a project RDL will hold the reference data required for a specific project.
The RDL is designed to be federated and as such can retrieve reference data from multiple sources and locations.
Part 4 is the main definition of all classes that can be used within ISO 15926.
Is this available online ?
Yes, there is an online version of ISO 15926 Part 4
or you can use a thin client to access the RDL
Will ISO 15926 Part 4 change ?
Yes fundamentally Part 4 will be appended to. However, existing items should never be changed, these will remain static once approved.