These are some brachos which fall in the middle between these two categories mentioned last week– that is – they aren’t clearly obligatory or not obligatory, and it is questionable if they are part of the 100 brachos or not:
Amen in response to the blessing on an Aliya to the Torah, or on the brachos of the haftorah: On the one hand, the listener is not personally obligated to recite these brachos, since he is not performing the mitzvah. On the other hand, the Torah reading and Haftorah are communal obligations. Some Rishonim count these brachos, and Mishna Berura (46:14) rules that if one has no other choice, these may be included in the one hundred brachos.
Amen in response to Chazaras Hashatz (the Chazan’s repetition of Shemonah Esrei): There are two ways to view these brachos; the listener has already recited Shemonah Esrei on his own, but nonetheless, Chazaras Hashatz is a communal obligation. For these reasons, the status is uncertain and the Mishna Berura 46:14 writes that it is questionable if answering amen to Chazaras Hashatz counts towards the one hundred brachos obligation.