Halliday & Matthiessen (2014: 55-6):
However, this theoretical template does not include details that are specific to particular languages or even to large sets of languages. For instance, the description of the system of MOOD is specific to English: according to this description, there is a system of INDICATIVE TYPE, with ‘indicative’ as its entry condition and ‘declarative’ and ‘interrogative’ as its two terms, and the term ‘declarative’ is realised by the sequence of Subject followed by Finite. This description is grounded in generalisations about English data, i.e. spoken and written texts; and all descriptions must be based on empirical evidence. … Typological generalisations are both possible and desirable, serving many purposes; but they are still grounded in empirical evidence, not based on theoretical hypotheses.