string-value, string-value-without-space, boolean-value, integer-value, double-value, time-value represent string, string without space, boolean, integer, float double and time (time is treated as a string without the format defined) respectively; these variables are considered as contents of XML elements. The part to the left of := in a BNF definition is taken as the tag name of the XML element. For example, <x-coord> ::= double-value, can be mapped as <x-coord>double_value</x-coord>.

<void-value> means the tag string can give a full definition and the element text content is set as "void-value". The reason for setting the element text content is that the element is required and needs to survive the procedure for removing empty nodes inside XML files.

The string to represent <object-name> must be unique inside the whole NDF file.

<version> takes the date of file format change as its version number. For example, if the latest change of file format was made on Dec 16th, 2014, then <version> is set as 20141216

<layer-defined-index> in <constant-layer-definition-objects> is referenced by xml nodes after it. For example, the camber definition uses the index number to indicate the layer to which the angle definition data is applied. The index number is based on sorting by layer span value and starts from 0. The layers to be sorted include constant span layers and user-defined layers from hub layer to shroud layer.

When there are multiple splitters in one blade row, it is suggested that these splitters are sorted by their offset angle to keep consistent.

<nurbs> supports trimmed NURBS curves; <bezier> does not support trimmed Bezier curves. Therefore the conversion from Bezier to NURBS is needed for any trimmed Bezier curve to be saved in NDF.