Halliday & Matthiessen (2014: 97):
MOOD is the major interpersonal system of the clause; it provides interactants involved in dialogue with the resources for giving or demanding a commodity, either information or goods-&-services – in other words, with the resources for enacting speech functions (speech acts) through the grammar of the clause: statements (giving information), questions (demanding information), offers (giving goods-&-services), and commands (demanding goods-&-services).