col C {
    box A; space 20; box B;
    arrows use label.pos=right;
    A@e->B@e {
        mark -12%: aaa [label.orient=tangent];
        mark 6%: bbb [label.orient=tangent_upside_down, label.pos=right];
        mark 50%: ccc [label.orient=tangent];
    }
}

space 50;

row R {
    box A; space 20; box B;
    arrows use label.pos=below;
    A@s->B@s {
        mark -12%: aaa [label.orient=tangent];
        mark 6%: bbb [label.orient=tangent_upside_down];
        mark 50%: ccc [label.orient=tangent];
    }
}