The NV Available to Sell refers to the maximum nominal value that you can currently sell.
The NV Available to Sell does not cover unsettled positions or pending sell orders, so there is a gap between your position value and the NV Available to Sell.
Unsettled positions can be sold once they are settled. Normally, your positions can be sold at T+2 (T refers to the transaction date. In the event of weekends or public holidays, sale will be postponed to the next trading day.)
For instance, if you hold $2,000 of Bond A on February 1st and you buy another $1,000 on the same day, your NV Available to Sell will remain $2,000 before February 2nd. If no sell order is placed from February 1st to February 2nd, your NV Available to Sell will become $3,000 on February 3th.