Rosin and acid core solders are manufactured with the solder wire as a tube and the center of the tube contains the flux. Rosin core solder contains a mild flux and is designed to solder parts where flux residue cannot be removed. An example would be electric circuit boards, or electrical connections. In these applications there is often no practical way to clean flux residue after soldering. Rosin flux residue is non-corrosive which eliminates the need for post solder cleaning. The mild nature of rosin core limits its use to copper and brass.
The name acid core tells us the flux is an aggressive type designed to solder steel as well as other metals. Flux residue from acid core solder must be removed after soldering to avoid potential part corrosion. Heat sources for cored solders can be air/fuel torches or soldering irons and guns.
If additional flux is needed during soldering it’s important to confirm the core flux type. Externally applied flux must be compatible with the flux cored composition you’re using.