Commentary for Ketubot 108:13

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:

One who sells and waives (her marriage contract) - sells her marriage contract or waives her marriage contract, waives and sells all of it (including the additional sum) and we don't say that the marriage contract is only the (basic) 100 (for a widow) 0r 200 (for a virgin):

For a rebellious (woman) - That we (will) learn in our chapter (further on 63a), that a woman that rebels against her husband [refusing to cohabit] etc., until when do we deduct [at the rate of 7 zuz per week], until the equivalent value of her marriage contract, (therefore) do not say the equivalent of the 100\200 alone rather we include the additional sum (to calculate the value of her marriage contract) how much we can deduct until it is all gone: