The depth of the function. As you go deeper into the stack, the depth gets smaller. For instance, if a script is in a wrapper function called from the main body of the stack, then the call stack comprises two levels, SE_STACK_INFO(0) and SE_STACK_INFO(1). The SE_SI_DEPTH is indication of how many function calls are nested beneath this level, including the level itself. In this case, the SE_SI_DEPTH of SE_STACK_INFO(0) will be 2, indicating 2 items nested. The SE_SI_DEPTH of SE_STACK_INFO(1) will be 1, as this function has only itself and nothing nested beneath it. The depth can be 0 if SE_STACK_INFO(0) is used while no code is executing and thus no function calls are nested at all.