If we want to create two payment terms like this:
L/C 90 days from shipment date
L/C 60 days from shipment date
On similar issue, it is seen that the above like terms are maintained just for a description only. No days are mentioned in the no. of days field in the payment term. Is it true ? How should we maintain the days viz. 90, 60 etc. in the payment terms ?

If shipment date =posting date , in OBB8 T.code as per your requirement create the Payment terms as per below Give the new variant and in sales text give it net 90 or net 60,in account type select customer or vendor which is applicable as Per your client. in default for baseline date select Posting date if =shipment date (Cross check with your client) and in Payment Terms under No. of Days give 90 and save. create another PT for 60 Days in the above way.

when we say a Payment is due on 90 days or 120 days, then, these days are from a certain date which is known as Baseline date in SAP. This baseline date can be configured in 4 ways:

1. Document date
2. Posting date
3. Entry Date
4. No Default : This means that the baseline date has to be entered manually by the user at the time of entry.

'We wrote On similar issue, it is seen that the above like terms are maintained just for a description only' so it is not a standard that it is maintained for description only. If  payment term is 90 days from the Shipment date, then the most important question is what is the shipment date and how will this be captured at the time of posting. If  posting date/Document date/entry date is the shipment date, then configure baseline date accordingly in OBB8 and then enter 90 days also, in this way system will calculate due date accordingly. If any of these dates are not the shipment date, then we need to derive the shipment date in Baseline date field by way of some exit and then due date will be calculated as Baseline date + 90 days