Your SQL is invoking the function through the inclusion of

fnGetPO([ON Order PO Info].[m1 po],[ON Order PO Info].[m2 PO],[ON Order PO
Info].[m3 po],[ON Order PO Info].[m4 po])

That means that the four fields from the table being passed to the function
are [m1 po], [m2 po], [m3 po] and [m4 po]. Since those fields are being
passed in that order to the function, within the function, po1 refers to the
value passed from [m1 po], p02 refers to the value passed from [m2 po], p03
refers to the value passed from [m3 po] and p04 refers to the value passed
from [m4 po]. strPO is simply an internal variable within the function. Once
it's been assigned a value, the function is assigned that value through the
statement

fnGetPO = strPO

so that's what gets back to the query.

The names used within functions really have nothing to do with what's being
passed to the function. The function could just as easily have been written
as either of the following and it would still accomplish exactly the same
thing.

Public Function fnGetPO(w, x, y, x) As String
Dim a As String

 If Not IsNull(w) Then
   a = w
 Else
   If Not IsNull(x) Then
     a = x
   Else
     If Not IsNull(y) Then
       a = y
     Else
       If Not IsNull(z) Then
         a = z
       Else
         a = "None"
       End If
     End If
   End If
 End If

 fnGetPO = a

End Function

or

Public Function fnGetPO(w, x, y, x) As String
Dim a As String

 If Not IsNull(w) Then
   a = w
 ElseIf Not IsNull(x) Then
   a = x
 ElseIf Not IsNull(y) Then
   a = y
 ElseIf Not IsNull(z) Then
   a = z
 Else
   a = "None"
 End If

 fnGetPO = a

End Function