METSIM has many "METSIM System Functions" such as the VXXX functions which work with stream data, and the FXXX functions which are mathematical functions.
These are snippets of code that have been packaged up and are made available. They do common tasks that are useful in building and using process models.
We can also write our own User Defined Functions, calling the METSIM System Functions when we want, and suddenly becoming geniuses at "Mineral Mathematics". OK, so maths is maths, it can be applied to anything. There is no such thing as Mineral Maths.
But METSIM is a system based on APL. And APL has been in continuous use as a mathematical notation and as a programming language since about 1968. So lots of people must have written lots of useful snippets of code, and the most commonly used should have ended up as part of APL for everyone to use? Well not in the modern world - there would be three lawyers and an accountant for everyone writing code and the IP Committe would decide being able to add two numbers was a corporate (or edu-corporate) strategic advantage and it should be removed from the syllabus and included in the commercialisation of funded research "Add Two Numbers" software.
But the APL language has indeed become more enriched over the years with System Functions that do just about everything we are likely to want to do beyond what METSIM provides "out of teh box" for the specific purpose of process modelling and simulation.
Many APL System Functions (not all of them) can be used to greatly enhance the power of METSIM enabling us all to stand on the shoulders of thousands of mathematical and data manipulation giants - most of whom would never have imagined what we can do with their code snippets.