Pledge is just a number in terms of quantity, FIFO will apply to date of purchase.
So when you sell 10, it’ll be the 10 from the first purchase and not the last 10
Every stock have unique identifier called ISIN no (This is true always)
Each share of a stock also have some unique identifier (Not sure about it)
3 . Pledge have list of shares (include Unique share identifier) already pledged.
Point 1 is correct. Point 2 and 3 are not as there is no such unique share identifier for each share of a scrip as shares get bought and sold through a trade ID/order ID and the ISIN is unique only to individual instruments.
Now, as per your initial example, if you sell 10 shares, only the free shares will be sold and you continue to get the collateral margin for the 100 shares pledged. But, the next day, when you will check your realized P&L and buy average of the 100 shares, it will be calculated as per the FIFO method.
So, technically there is no change in the pledged quantity and you receive the credit for the sale of 10 shares as per your sell price only but the P&L and the buy average gets computed as per FIFO method.