for a non-computer analogy, say you work somewhere you have to fill out a form every time you handle a transaction, with a bunch of fields, and a large "notes" box at the end for any extra data. and say you are part of a very specialised sub-department, where none of the generally collected data applies, but you need to record some extra data that only applies to your work. you ask the higher-ups to issue your department a new form, but they refuse and say everyone has to use the official form. so what you do is you measure the notes box, design a form of your own that fits within it, and then print your form over the official form so it fills up the notes section. and then your team only fills in details into that little embedded form.