NO_OUT is useful when you want to hide the field on the output, but still allow users to unhide it on demand. As lots of fields from field catalog, it can be filled with ‘X’ or space (default).
If ...
Technical field allows you to do similar thing as NO_OUT, you can hide the field on the output, the difference is that, users are not able to unhide this field. As NO_OUT, in order that TECH field work, ...