2c389a02__Specifications___openEHR_wiki__openEHR_equivalent
[Code mnemonic] DER [Rubric] derived [Description] An actual value may exist, but it must be derived from the provided information (usually an expression is provided directly). [openEHR equivalent] 
This can happen anywhere in the data; openEHR doesn't see it as missing data / null data. The archetype shows what is derived and what is not, e.g. Apgar total from input values. Maybe; but in some cases the value is derived from other values (which may not yet be known) using an expression, and in other cases it's given directly. I guess that in OpenEHR you would just model that directly, but it seems to me to be confusing the process of getting the value with the semantics of the values themselves.
Null Flavours and Boolean data in openEHR - Specifications - openEHR wiki
openEHR equivalent