True if the LDL value is calculated from Chol, Trig, and HDLC.

It's definitely better that whether the LDL was calculated or not comes from the LOINC code. But this is defined to exercise the extension machinery in the publication tooling, and to make this comment. Actually, the location is odd too - it would probably be on the root observation, but this is for testing purposes.

unique id for the element within a resource (for internal references).

An Extension

Source of the definition for the extension code - a logical name or a URL.

The definition may point directly to a computable or human-readable definition of the extensibility codes, or it may be a logical URI as declared in some other specification. The definition should be version specific. This will ideally be the URI for the Resource Profile defining the extension, with the code for the extension after a #.

Value of extension - may be a resource or one of a constrained set of the data types (see Extensibility in the spec for list).