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According the Srauta sutras, there are seven Haviryajnas. Agnihotram is involved in many of these yajnas.
As given below according to सुदर्शनसूरेः तात्पर्यदर्शनम् गृह्यतात्पर्यदर्शनव्याख्या १.१ of Aapasthamba Grhyasutras there are 7 Paakayajnas
तत्र च सप्त पाकयज्ञसंस्थाः औपासनहोमो, वैश्वदेवं, पार्वणमष्टका, मासिश्राद्धं, सर्पबलिरीशानबलिरिति ।
There are 7 Paakayajnas - Aupaasanahoma, Vaisvedevam, Paarvana, Astaka, Maasisraddha, Sarpabali and Ishanabali.
They involve offering of cooked food (paaka) as Havis to devatas.
सप्तैव च सोमसंस्थाःग्निष्टोमोऽत्याग्निष्टोम उक्थ्यष्योडशी वाजपेयोऽतिरात्रोऽप्तोर्याम इति ।
Seven are the Somayajnas - Agnishtoma, Atyagnisthoma, Uktya, Shodasi, Vaajapeya, Atiratri, Aptyoryama.
These sacrifices involved the extraction of Soma and its offering to Soma, Indra and their companion gods. Examples of Soma sacrifices are Agnistoma, atyagnistoma, uktya, shodasi, vājapeya etc.
सप्त च हविर्यज्ञसंस्थाःग्निहोत्रं, दर्शपूर्णमासा, वाग्रयणं, चातुर्मास्यानि, निरूढपशुबन्धस्, सौत्रामणिः, पिण्डपितृयज्ञादयो दर्वीहोमा इति । (५)
Seven are the Haviryajnas - Agnihotram, Darsapurnamaasa, Chaaturmaasyas, Niroodha pashubandha, Sautramani, Pindapitruhagnas, Darvihomaas.
In Haviryajnas the yajamani or priests pour oblations of milk, vegetable oil, sesame oil, clarified butter, honey and other liquid offerings.
They are performed daily by the householders and are given in Taittiriya Aranyaka (2.10) apart from the Aapasthamba Grhyasutras.
These were meant to mark the progress of a student’s education under a teacher and his success in attaining the knowledge of the Vedas, while keeping his vows of celibacy and secrecy until his graduation as a Snataka (one who has bathed and was ready for the life of a householder or an apprentice).
They are performed at different times in the life of an individual and involve homas and istis.