pdaExpression
Expression need to be commonly evaluated by many software, so having a restricted set of math / types and so on is key. In PDAFlow lib2/expr there is a yacc/lex parser available with some unit support as well as double / complex expressions. An alternative is tinyexpr, but this is more restrictive, so may be very unhandy for things like waveguide model expressions.
Structure
This element is simple - it does not have attributes or elements. The text that fills up the... is value used.
<stddev> ... </stddev>
XSD The schema file can be downloaded or viewed at xPDK_Base.