Please try to make the question title more consistent with the actual question. Looking at the title, most people would think this is regarding a general career advice, not a technical question. You simply won't get many replies with a generic title like that.
The question itself is not clear. At first you're asking about "line item in sales order" but then it's about schedule lines. These are two completely different things. Schedule lines are also driven by the product availability (ATP) and normally don't need to be (and shouldn't be) manipulated programmatically. If there is some peculiar ATP requirement then there are designated user exits for ATP which will then trigger scheduling accordingly.
Direct this question back to the SD consultant, ask them what is the business reason for this and why do they believe this requires a user exit. They would also need to write a specification for you. So far the business requirement itself seems questionable to me.
In general, there are hundreds of SCN posts about user exits that can be easily found in Google by using keywords "user exit" + whatever you want to do + site:sap.com. There is also SAP documentation and even an SAP Press book specifically about SD enhancements.
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