They mean the stack depth is guaranteed to be statically apparent at each point in the control flow. In the JVM the class loader checks this with an analysis ph

They mean the stack depth is guaranteed to be statically apparent at each point in the control flow. In the JVM the class loader checks this with an analysis phase during loading. (The JVM design makes this harder than it really needs to be, since it doesn't insist on reducible control flow, iirc. You could say OTOH that irreducible control flow is more valuable than trivial analyzability, which is a reasonable take, but conflicts with the above comment's design point of a stack machine being both very simple for an interpreter and not an obstacle to quickly compiling to decent register-machine code. Needing a fancier stack-depth analysis would be that sort of obstacle.)