In (this book, in the sūtras relating to) compound, the word that is exhibited in the Nominative -1st case, is called upasarjana or the secondary word.,
This defines or rather describes the Upasarjana. In the sutras referring to compounds, the words that have the case termination of the nominative, are called Upasarjanam.
Thus sutra 2.1.24 (द्वितीया श्रितातीत पतितगतात्यस्तप्राप्तापन्नैः) is a samasa sutra enjoining composition. In this sutra the word द्वितीया is exhibited in the nominative case, and the remaining words are in the Instrumental case. The sutra means: \An accusative (द्वितीया) is compounded with the words श्रित, अतीत, पतित, गत, अत्यस्त, प्राप्त, and आपन्न and forms Tat-purusha\. Therefore, in forming accusative compounds, the word having the accusative case will be called Upasarjana. Thus कष्टं + श्रितः = कष्टश्रितः. Here the word कष्टं being Upasarjana, stands first in the compound, by Rule 2.2.30. उपसर्जनं पूर्वम्, (in a compound the Upasarjana should be placed first). Similarly in rules relating to the formation of instrumental, dative, ablative, genitive and locative Tat-purusha, the words standing in those cases will be Upasarjana. Thus शंकुलाखण्डः, यूपदारु, वृकभयम्, राजपुरुषः, अक्षशौण्डः.
Upasarjana is, therefore, that word which either by composition or derivation loses its original independent character, while it also determines the sense of another word called प्रधान or principal.,