Derived attributes & associations are read-only, but you can also have writable associations that update other attributes or links when they themselves are set.

Read here about how that works on attributes: Derived settable attributes.

OCL for reading Class2selection:

self.Class2

EAL for the writing Class2Selection:

self.Class2 := vInputParameter;
self.SelectionLog := self.SelectionLog + DateTime.Now.asString + ': '+ vInputParameter.Name + '\r\n'

The most common use case for this is to execute "extra" code for recalculating other persisted values based on the new value of a change to a link.

Here is the sample model to be loaded into the designer: File:DerivedSettableExample.modlr