Mood Element vs MOOD System

Halliday & Matthiessen (2014: 142n):

Note the distinction in capitalisation between ‘Mood’ as the name of an element of the interpersonal structure of the clause (Mood + Residue) and ‘MOOD’ as the name of the primary interpersonal system of the clause — the grammaticalisation of the semantic system of SPEECH FUNCTION. This follows the general convention whereby names of structural functions are spelt with an initial capital and names of systems with all small caps or upper case.