Just a wee note to help you and that came as a result of a question…
Internal factors relate to choices the producer makes for their own reasons or that they have control over. (e.g.: Band image/producer’s target audience/casting/budget/etc)
External factors relate to things outwith the control of the producer but which the producer has to bear in mind such as Codes of Practice (from CAP), working conditions must comply with relevant legislation (e.g.: Health and Safety), regulatory bodies (e.g.: BBFC/ASA/the Law!), and so on…
In general terms, when you made the poster, you had internal factors to consider such as actors (who you have control over), budget, software, lighting, equipment, etc… but while making it, you have also had to be aware of (and possibly made changes to comply with) external factors such as making sure your poster is legal, decent, honest and truthful… etc.
So in short… an internal factor is one that a producer controls, and external factor is one that someone else controls… yes, they sometimes cross over, but keep it simple… stick with the obvious!
Final example from PQD:
Internal: The producers choose the cast (Terence Stamp)
External: The BBFC issue the classification certificate (15)
And a cross over: The producers approve the script (and allow bad words) which is an internal factor but as a result of their decisions, the BBFC give it a 15 certificate which is an external factor.