This question is discussing the field Diag code / Diag code pointer or what is normally know as box
24 E on the 1500 form (Diagnosis Pointer).

In the software as on the 1500 form each transaction can have upto 4 diagnosis here is an example
1500 FORM
Box 21 DIAGNOSIS OR NATURE OF ILLNESS
1.    250.00                          3. 122.9
2.    V42.0                            4.  475.00

In box 24E Diagnosis pointer on the 1500 form, you are entering which of the diagnosis above are to be used for this specific line item or procedure.

So if I am dispensing a Glucometer in my procedure line i would enter a 1 (number 1) into the Box 24E Diagnosis pointer field to say that diagnosis 250.00 is to be used with this Glucometer procefure.

Other ways of using Box24E would be
1-2      both diags 1 and 2 are being used
2-3      both diags 2 and 3 are being used
1-3      diags 1,2,3 are being used
1-4      all 4 diags are being used
3-4      diags 3 and 4 are being used
NOTE:
If you leave box 24E blank the software will automatically put a 1 into the field.