Halliday & Matthiessen (2014: 35):
Still, while field, tenor and mode resonate with semantic systems in the first instance, they do penetrate into lexicogrammar: field values put ideational wordings at risk, tenor values put interpersonal wordings at risk, and mode values put textual wordings at risk. … Indeed, the tenor variables of power and contact may be grammaticalised as part of the core interpersonal system of mood in a language, as in Japanese and Korean.