A screen that DOES NOT EXIST, for users who SHOULD NOT SEE THE FULL ONE, over data from MORE THAN ONE SYSTEM. IF NONE OF THAT APPLIES, SAY SO.
Those three are the genuine cases, and a request that matches none of them almost certainly has a cheaper answer.
A SCREEN THAT DOES NOT EXIST IS THE CLEAREST. The business has a process Fusion does not model — an approval with their own steps, a form for something the standard application has no concept of. There is nothing to personalise, because there is nothing there.
USERS WHO SHOULD NOT SEE THE FULL SCREEN IS THE SECOND, AND IT IS STRONGER WHEN THEY ARE NOT FUSION USERS AT ALL. A warehouse operative, a field engineer, an external partner — people who need to do one thing and should not be given a Fusion licence and a sixty-field page. That is a real application, and hiding fields on the standard screen does not solve it.
DATA FROM MORE THAN ONE SYSTEM IS THE THIRD AND THE ONE PERSONALISATION CANNOT REACH AT ALL. A page showing a customer's Fusion orders alongside their support tickets from somewhere else. Page Composer can rearrange a Fusion page; it cannot put another system's data on it.
AND "IF NONE OF THAT APPLIES, SAY SO" IS THE INSTRUCTION, BECAUSE IT IS THE PART THAT IS PROFESSIONALLY HARD. The client has asked for an application, somebody would be paid to build it, and the honest answer is that Page Composer does this in an afternoon.
SAYING IT IS BETTER FOR EVERYBODY INCLUDING YOU, AND THE REASON IS LESSON 6. An application built where one was not needed is a maintenance obligation the client did not know they were buying — and the conversation about why it broke after a quarterly update is one you will be having, about something that did not need to exist.
SO ASK THE THREE QUESTIONS BEFORE ESTIMATING ANYTHING. Does the screen exist? Who is using it? Where does the data come from? Three answers, and they settle it.