These videos should help.

Customer Codes
Sequential Numbers
Sequential Annual Coding

Thanks! I have seen these videos previously also but not able to figure out the correct way to do so! I will watch them again!

Try a combination of customer code and Letters....  D and P followed by a number.  what do you think?  Kevin is on the money just got to think outside of the box.

Dear Juan

Could you please explain me how to assign these Letters automatically to a field?

Well nothing is automatic you need to set it up I do a combo box with D and P then when the customer code generates copy both the combo and customer code to your field. If you set it up to execute after update then make your combo and customercode unbound
combo P + customer code
txtfield = DepartmentCombo & CustomerCode

Value List Combo  This link will show you how to set up the combo box

Concatenation this will show you how to concatenat the fields

Get a Value from an Open Form Field

I think this will help if not let me know.  

V/r
Juan

Hi Kashif, I think the "Sequential Annual Coding" video Juan mentioned should tell you everything you need.  If you want Dept A's project numbers to be A0001, A0002, etc., and Dept B's project nums to be B0001, B0002, etc., and so on, then just substitute the years used in Richard's video with A, B, etc.  These videos don't tell you exactly what you need.  You need to be able to adapt the ideas from these videos to your exact specifications.

You are missing a space before you And (second line).

can you please let me know what am I doing wrong? I tried to fill a data in a combo box and trying to generate a relation code.

also in another code I tried saving a text field from the value box. and then trying to concatenate with a value of the customer code but wasn't successful there as well.

Kindly help

@Kevin Robertson did the space still no luck!

Remove the parentheses from RelationTypeID.

Current SQL for ref:
X = DCount("*", "CustomerT", "RelationTypeID=" & RelationTypeID & " And Not IsNull(RelationCode) and CustomerID <>" & CustomerID)

If you harcode some values does it work?

X = DCount("*", "CustomerT", "RelationTypeID=1 And Not IsNull(RelationCode) and CustomerID <> 2")

Replace 1 and 2 with values that exist in your db.

yes that works but I need options to compare them with multiple option

Can you print the variables so you know they are correct before being passed to the DCOUNT.

sorry I did not understand what you meant!

Debug.Print RelationTypeID
Debug.Print CustomerID

or

MsgBox RelationTypeID
MsgBox CustomerID