When using built-in functions, Excel provides remarks. For instance, if you
type =sumif( and then click the fx button (insert function), you will see
suggestions to the right of each input box. Notice range says "reference",
criteria says "any", etc. and just below the inputs is an instruction that
says "adds the cells specified by a given condition or criteria". And below
this, the instruction changes depending on which box you hilite.

How can I fill these fields with instructions? Below is a simple example
another person wrote. For instance, I'd let the user know that ACFM s Actual
Cubic Feet per Minute, etc. And I might have an overall instruction, "This
function calculates standard cubic feet per minute".

Public Function SCFM(ACFM As Double, TempF As Double, PSIA As Double)
'ACFM = Actual Cubic Feet per Minute
'TempF = Temperature (°F)
'PSIA = Pressure (PSIA)

'Constant Definition
T0 = 459.7
TS = 70 + 459.7

'Other
TA = TempF + T0

PA = PSIA
PS = 14.7

SCFM = ACFM * (TS / TA) / (PS / PA)

End Function