विभाषा दिक्समासे बहुव्रीहौ

The above words are optionally सर्वनाम when they occur in a बहुव्रीही compound signifying direction -- दिङ्नामान्यन्तराले 2.2.26.
This is an exception to sutra 1.1.29 which follows. As a general rule (see sutra 1.1.29), in bahuvrihi compounds, these words sarva &c. are declined like ordinary nouns. The present sutra declares an option to this, in special cases relating to compounds signifying direction. Thus the bahuvrihi compound of direction उत्तरपूर्वा 'north-east' may form its dative case as a pronominal, e.g. उत्तरपूर्वस्यै or it may form its dative like ordinary words ending in आ, i. e. उत्तरपूर्वायै. So दक्षिणपूर्वस्यै or दक्षिणपूर्वायै.

Why do we say "when signifying a direction"? The next sutra will enjoin that in bahuvrihi compounds, these words are not treated as pronominals. Therefore, if in the present sutra, we had not used the word दिक्, it would have been impossible to know where there was option and where prohibition. By employing the word "direction," it is thus determined, that in a samasa, relating to direction 2.2.26, there is allowed option, while there is prohibition in all other kinds of bahuvrihi compounds.

Why do we say "in compound"? So that the option may be in that bahuvrihi which is purely samasa, and in no other bahuvrihi. There is no option allowed in that bahuvrihi where there is a semblance of bahuvrihi but not exact bahuvrihi.

The quasi-bahuvrihi, or analogical bahuvrihi compounds, or to use the Sanskrit phrase, bahuvrihi-vadbhava, is taught in sutra 8.1.9. where words in repetition are treated like bahuvrihi. Thus एकं + एकं = एकैकम् 'one and one, each one.' Bahuvrihi compounds are possessive compounds, but in the above example, no idea of possession enters. However the compound here is treated like a bahuvrihi, for the purposes of the elision of the case-affix of the first member, and the whole compound gets the designation of pratipadika. In compounds like this, therefore, which are bahuvrihi-vat, but not exactly bahuvrihi, the present rule and the succeeding sutra do not apply. Thus एकैकस्मै देहि 'give to each one,' दक्षिणदक्षिणस्यै देहि ॥

Why do we say "in the bahuvrihi"? So that there may be no option in the dvandva compound. Thus दक्षिणोत्तरपूर्वाणाम् 'of the south, north and east.' The prohibition in the case of dvandva by rule 31 is absolute and invariable.