The main focus of the Mīmāṃsā is the exegesis of Sacred Texts, especially of their sacrificial portion, i.e., the Brāhmaṇas. There is already enough literature about how to perform a sacrifice (apart from oral instructions, practical manuals are called paddhatis or prayogas) and Mīmāṃsā authors point instead at systematising the mass of Vedic texts. Their systematisation is highly hierarchical and centripetal and has as its centre the sacrificial prescription. All the rest (narrative passages about myths, mantras, Upaniṣadic teachings, etc.) only makes sense –according to Mīmāṃsakas– insofar as it is connected to a prescription.
The standard Mīmāṃsā example of prescription is svargakāmo yajeta ("the one desirous of heaven should sacrifice"). To it an instrument, a result and a procedure are connected. And all ritual acts are classified in relation to it, so that at the end one can figure out a coherent whole ordered from the top (the prescription) until the minutest details.
The need for classification regards also prescriptions themselves. In fact, structuring the prescriptive component of a text allows one to better understand its hierarchical links. For instance, principal prescriptions (such as the one mentioned above) are to be distinguished from subordinate ones (such as the ones enjoining presacrifices or prescribing the ritual substances).
Since the Veda is authorless, the only possible perspectives on the text are the hearer's one and the text's inner one. Accordingly, at least two classifications of prescriptions are possible. The first one is: utpattividhi/viniyogavidhi/adhikāravidhi/prayogavidhi, the second one: apūrvavidhi/niyamavidhi/parisaṁkhyāvidhi. See previous posts for details.
The first classification expresses the role of each prescription and defines it –in Mīmāṃsā terminology– "according to its own nature" (svarūpābhidhāna). The latter conforms to the role of a prescription within a text from the point of view of the hearer. If it conveys something utterly new for the hearer, then it is an apūrvavidhi. If something partly new, it is a niyamavidhi. If it looks like a positive injunction but is instead to be interpreted as a prohibition, it is a parisaṁkhyāvidhi. The latter case, as already hinted at, is one Mīmāṃsā authors generally try to avoid.
(on the right: simplified map of the sacrificial area. From Jan Houben, www.jyotistoma.nl).