The meaning of paribhāṣā is –against expectations– not fixed. As for its usage in the Śrautasūtras, Chakrabarti explains:
The term paribhāṣā was not well-defined and it appears that no definition was strictly adhered to when the sūtras were characterizes as paribhāṣā. Not only the basic interpretative clues, but also the general rules were regarded as paribhāṣā. They contain some heterogeneous topics, and some insignificant rules too crept into the paribhāṣās (Chabrabarti1980, p. vii).
As for Grammar, Dominik Wujastyk (editor of the Vyāḍīyaparibhāṣāvṛtti) argues that paribhāṣās have been introduced for solving problems of the Aṣṭādhyāyī and suggests that they might have, accordingly, a different degree of abstraction:
Rather than giving up Pāṇini's grammar as wrong in such cases, it is natural to try to improve the theory. The tradition introduces extra rules to correct the situation. These are the paribhāṣās, a term which may be translated as 'metarules', 'principles', 'theorems' or 'auxiliary hypothesis' (Wujastyk1993, p. ix).
In Mīmāṃsā, by and large, we might understand the term paribhāṣā (and even more so its quasi-equivalent in Mīmāṃsā, i.e., nyāya) in two senses: in a loose or in a technical sense. In the looser sense, a nyāya is a general rule regarding a certain behaviour. In the stricter sense, it is a rule ruling other rules. An example of the former kind of nyāya is the Mīmāṃsaka khalekapotavan-nyāya 'the rule of the pigeons in the threshing floor'. This is only a similarity used to represent cases in which many items at once occur in the same place, just like pigeons hurrying to grasp some grains. But it does not regard rules. By contrast, rules such as 'the meanings of the words in the Mīmāṃsāsūtra are the same as in the ordinary communication' (in Śābarabhāṣya ad Mīmāṃsāsūtra 1.1.1) is a rule which applies to other rules, the ones mentioned in the MS.
Although the technical usage of nyāyas derives from the looser one, it is convenient to distinguish between the two. In order to be a meta-rule, a rule needs to refer to further rules. Since the main focus of the Mīmāṃsā is the Veda, rules regarding it directly do not need to be meta-rules. By contrast, meta-rules are rules ruling a certain exegetical rule. For instance, all rules applying to other rules of the Mīmāṃsāsūtra, or all rules applying to an exegetical rule discussed in the ritualistic thought prior to the Mīmāṃsāsūtra.
For other cases testifying a common prehistory shared by Śrautasūtras, Grammar and Mīmāṃsā authors, see this post (on tantra), this and this one (on prasaṅga, the latter discusses a metarule, i.e., "an exception counts more than the general rule").