In a case where the sense is that of a taddhita-affix or when an additional member comes after the compound or when an aggregate is to be expressed, then a word signifying a point of the compass or a number, enters into composition with a case-inflected word which is in agreement with it by being in the same case and the resulting compound is tat-puruṣa ,
First let us take the case where the sense is that of a Taddhita-affix. Thus when a compound is formed out of the words पूर्वस्यां शालायां भवः 'that which is in the eastern hall' (in which analytical exposition of the compound in question, the word भव 4.2.107 serves to represent the force of a Taddhita -affix) the compound having reached the form of पूर्वा+शाला, the feminine termination of the पूर्वा is rejected, because Patanjali declares that the masculine state belongs to a pronominal when exercising any of the five functions belonging to a word; we have :-पूर्व + शाला + अ 4.2.107 = पौर्वशालः (VII. 2. 117 and VI. 4.148) 'who is in the eastern hall.'
So also when an additional member comes after the compound (uttarpade) As, पूर्वशालप्रियः or अपरशालप्रियः 'loving the eastern or western hall.' Those Uttarpada Compounds are invariable (Nitya) compounds, so that they can not be resolved into their component elements.
When a समाहार 'aggregate' is to be expressed, it is of course impossible to use a word expressing points of the compass ( दिक् words ). The following compounds are formed with numbers (सङ्ख्या), when employed with the force of a Taddhita affix; as, पाञ्चनापितिः 'relating to five barbers.' पाञ्चकपाल 'relating to an oblation offered in five cups' (कपाल) 4.1.88. So also पञ्चगवधन 5.4.92 'Whose wealth consists of five cows,' दशगवधनः
The following are examples of aggregates:- पञ्चफली 'the collection of five fruits' 4.1.21 दशपूली 'an aggregate of ten bundles,'त्रिलोकी 'the aggregate of the three worlds.' पञ्चकुमारि 'an aggregate of five virgins;' this word is neuter by (II. 417.)and the long ई is shortened by (I. 2. 47.),