1. (see e.g. Jaimini, MS 1.1.2, 1.1.4 and 1.1.5 and Śabara's commentary thereon) as an epistemological terminus technicus referring to our sources of information: perception, inference, communication…
2. (see Śabara on 1.2.31, 1.2.46…) as a Mīmāṃsā terminus technicus referring to a source of information about an exegetical point.
In the latter sense, a pramāṇa may be the direct mention of a term in a certain connection, or its indirect indication through number, gender, etc. For instance, the direct mention of kuśa grass as sacrificial substance allows one to conclude that it has to be applied (viniyoga) to the sacrifice.
The second meaning seems to me the original Mīmāṃsā one. It might have been older than the systematic discussions about pramāṇa in Indian epistemology (which also rarely predate Kumārila and Dharmakīrti). So, exegesis might have been the way through which Mīmāṃsā elaborated a philosophical lexicon. Similarly, V. Eltschinger argued that the Buddhist epistemological school of Dharmakīrti derived its lexicon and interests out of the Bodhisattva-bhūmi literature discussing internal Buddhist problems (antaḥvidyā, knowledge of Buddhist texts).