Hi Sanjay, you use ABNAN to carry out post capitaliation of an asset that was put into service in prior year. ABNAN can also post to asset with value date in current year. But its primary use is to let you capitalize an asset in the current year while catching up the prior year depreciation.
ABAWN if for revaluing your assets. You provide what new value you would like your asset to have. The key difference is that ABAWN, AFAIK, can only revalue your asset in the current year with possible depreciation write back (or not) because your asset value date as to mirror your fiscal year...meaning, unless you are opening prior periods, then currenty year is your option. ABNAN, on the other hand, would allow you to use asset value date as far back as the post capitalization date, though your actual posting is still in current periods (SAP does the necessary adjustments to catch up or smooth)
hope this helps,
Sam