I am assuming that you are referring to a value list and not a list in a portal or list view layout.
This is called a conditional value list and if you can set up a relationship that matches by the customer ID, you can limit the list to only the values from records that match by that customer ID.
I call this a "basic conditional value list" as it's the simplest and most commonly used, but have figured out 9 other ways to set up such a conditional value list.
See this file: "Adventures In FileMaking #1 - Conditional Value Lists". This free to download database file demonstrates all 10 methods with working examples and detailed descriptions of how each works and how it was set up.
Thanks Phil, I use your "Adventures in Filemaking" often. Actually I'm referring to a layout in "List View"
Then you simply need perform a find for the desired records in your list view and yes a global field is often a good choice for that. For examples of scripted finds that start from criteria entered into global fields while in browse mode, see: Scripted Find Examples
But sometimes this criterion is not entered by the user but captured in a variable from the current record by a set variable script step before entering into find mode. This is much the same scripted method, but your set field steps refer to the value of the variable instead of the global field.