"Dimension" refers to units: in what units should volume be expressed.
Stocks are traded in shares, so you just give the quantity of shares that have been traded.
For bonds the "nominal" is also known as the Face Value (also sometimes called the Par Value, at least in the case of coupon bonds). This is the amount in dollars (or other currency) for which the bonds will be redeemed at maturity if all goes well (if there is no default). In the US, at least, a standard unit for measuring bond volume is "thousands of dollars of par/face/nominal value". The convention is that "1 bond" or "1 lot" corresponds to 1000 USD of face value, and you are not allowed to trade less than 1 lot.