To connect HTTP requests with the responsible ABAP applications, HTTP request handlers are used. Technically, these handlers are methods of ABAP classes, assigned to SICF nodes.
Handler determination happens in a top-down direction, so from parent node to child node in the SICF tree structure. First the handler(s) of the parent node will be called, and then the handler(s) of the child node may or may not be called, depending on the value of the field if_http_extension~flow_rc. The same logic applies if one node has several handlers - first the topmost handler will be called, and then the rest of the handlers may be called from top to bottom, depending on the value of if_http_extension~flow_rc.