Halliday & Matthiessen (2014: 97):
Every free clause selects for mood. … A free major clause is either indicative (giving or demanding information) or imperative (demanding goods-&-services) in mood; if indicative, it is either declarative (giving information) or interrogative (demanding information); if interrogative, it is either ‘yes/no’ interrogative or ‘WH-’ interrogative.