For floodsub, what would be a proper topic string?

If I use libp2p + floodsub to build an app, it is a bad idea to use a generic string, such as “message”, as the topic?

will that cause conflict with other apps built on top of libp2p + floodsub (and also uses “message” as the topic string)?

Should I give each individual function a separate topic string, for example:

or do I simply use one single topic for the entire app?

Will floodsub be vulnerable to DDOS? for example someone sends garbage using the same topic string to all peers?