"I use @FormFieldValue function.
Here is some old code from a subform.  It is in OnFormEntry section:

It is a subform that is naturally linked to ClientNumber, but I like to have an element with the "Key Value" on the subform also.  On the subform, ClientNumber is ReadOnly.  The Parent form is named frmClients."

Bob, I 'add a new record in Purch (the Parent), then tab into POLines (the child subform) to add records of products (line items) to be ordered.  I have an "Add a New Line Item" command button in my subform that saves the current subform record, and creates a new record so I can enter an new line item.  

the PONum programming to link the parent po# with the child PONum field works fine on the first subform record, but it doesn't fire on additional subform records.  
         With what I'm trying to do, I'm think that I should be using a 'record' function instead of a 'form' function? .... so that each line item (subform record) fires its own po# value in the PONum field.

UPDATE:
I added the PONum = FormFieldValue("Purch", "PONum", 0) to the ItemNum 'on element change' , and it seems to work reliably ... am I missing a pitfall somewhere?

IF @isNew then
     
     {
           XLOOKUP (@fn, ItemNum, "INVENTORY!ItemNum", "ItemDesc", ItemDesc)
           XLOOKUP (@fn, ItemNum, "INVENTORY!ItemNum", "ItemCurrentCost", ItemCost)
           XLOOKUP (@fn, ItemNum, "INVENTORY!ItemNum", "ItemUoM", ItemUoM)
           PONum = @FORMFIELDVALUE("Purch","PONum",0)
     }

Bob,  forum protocol acknowledged!

Tomorrow I will post the code I used to generate new line items in my subform, in the event I'm wandering down the wrong road (not using it standalone now, but appreciate the code help to prevent standalone use).

The sample I had provided was based on being in the Parent Form and moving the cursor into the sub form creating records in sequence.  The code was triggered by OnFormEnter programming i the subform.  But, by using @CreateNewRecord, that is like opening the sub form in standalone mode, but you are not actually opening the sub form so that code does not apply.

If using @CreateNewRecord you should probably include some code in the button that triggers this.  You should probably get the current PO number from the current Purchase Order, and make it into a Global Variable.  Then use the Global value to put into the PO number element on the subform vs. using FormFieldValue from the Parent form.

I don't know how this is all structured, where your buttons are, etc.  They coudl be on the Parent form, could be on the POLine subform, could be on any other form, could be a Menu Option, etc.  Could be more than one that do the same thing.  I am concerned that you could start on one PO, perhaps close the parent or navigate to a different form/PO by mistake, and click a button to add a PO LIne.  so, the original PO might not be the current one.  Not knowing your design, I can't tell you when to save the PO as a global, but I would suspect when OnFormEnter you update the Global Value if you decide to use one.  

My designs don't use a button to Add a Record,  I just enter data into a field on the subform and it is created.  Go to the next record and add data into a filed, it is created.  It can have defaults, auto fills, dittos, etc.  No need to click on a button.  Just type data, press the down arrow if in Table View, and start typing into the next record.  Creating records this way guarantees you are in the correct PO form and you can use FormFieldValue safely because the subrecord is linked to the parent record.

We could discuss designs forever and there is no "right" answer, but thanks for the picture.
I have included it here, better to see in message vs. need to jump to another web page.
(Edit: A replacement image was placed here on 3/6/09 per request of lksseven).
 

I am no sure that you still have a problem, I see the PO number on the subform.  I thnk we got sidetracked about comparing GlobalValue vs. FormFieldValue.

It looks like you have the Add PO Line button on your subform.  If that is using CreateARecord then the programming in OnFormEnter will not be executed when the record is created.  You will have to put the code into the button that would normally go into OnFormEnter.  If you do that, you can use FormFieldValue vs. GlobalValue, easier to code, only needed on subform.  

So, problem solved?