Halliday & Matthiessen (2014: 51):
But ‘text’ is a complex notion. In the form that we typically receive it, as spoken and written discourse, a text is the product of two processes combined: instantiation and realisation. The defining criterion is instantiation: text as instance. But realisation comes in because what becomes accessible to us is the text as realised in sound or writing. We cannot directly access instances of language at higher strata — as selections in meaning, or even in wording. But it is perhaps helpful to recognise that we can produce text in this way, for ourselves, if we compose some verse or other discourse inside our heads. If you ‘say it to yourself’, you can get the idea of text as instance without the additional property of realisation.