פירוש על כתובות 108:9

GM' Wants 'to write' is not (the terminology) mentioned (in our Mishna) - Had the terminology (lit. learned) 'to write' (been used in the Mishna), we would not have deduced that the Toisefet (additional monies) are called Ketuboh (marriage contract) rather a gift given willingly, it would not have the legal status (lit. name or label) of a Ketuboh , now that (the Mishna) uses (lit. learns) the expression Lehoisif (to add) it implies that it is additional to the Ketuboh, that the Rabbi's (lit. wise men) instituted, and has the legal status (lit name or label) of Ketuboh. Also he (Reb Aivu) supports it (this understanding) etc.:

Conditions of a Ketuboh (marriage contract) - Additional (funds) that are additional gifted to her, and similarly food and all those (various other conditional requirements of a Ketuboh) that were learnt in the chapter above: