PURPORT

Srila Rupa Gosvami has explained kama-likhana in his book Ujjvala-nilamani (Vipralambha-prakarana 26):

sa lekhah kama-lekhah syat
yah sva-prema-prakasakah
yuvatya yuni yuna ca
yuvatyam samprahiyate

"Exchanges of letters between a young boy and young girl concerning their awakening of attachment for one another are called kama-lekha."