Scripted pipeline dynamic library version clarification question

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Dynamic versions for pipeline libraries is supported with the library step:

// A Jenkins file
library "name@${VERSION}"
doSomething()

Where doSomething() is implemented in vars/doSomething.groovy and forms a custom step.

What is not clear to me in the Loading libraries dynamically documentation is this sentence:

Using classes from the src/ directory is also possible, but trickier.

There are THREE places where this statement could apply:

  1. In the Jenkinsfile - as it may need to import classes in src/
  2. In the vars/doSomething.groovy global variable (aka “custom step”) - as they may need to import classes in src/
  3. In the groovy files under src/ - as they may need to import classes from src/

So when it says “trickier”, where is it trickier? In all three places?