A zone is the place where two lithospheric plates converge, one riding over the other. Most volcanoes on land occur parallel to the boundary between the two plates.

Ocean-Ocean Convergence - Oceanic lithosphere is relatively dense and can easily sink into the Earth's mantle. When two oceanic lithospheres collide, one runs over the other which causes the latter to sink into the mantle along a zone called a subduction zone.

Explanation:

https://personal.ems.psu.edu/~jte2/geosc20/lect28.html