For example, if you have $2,000 in your Autoinvest cash, you will find that after an autoinvestment you are left with some spare cash in that account (e.g. $5.50).

The primary reason for this is that shares aren't fractional - i.e. you have to buy a whole number of shares each worth a specific value at the time you buy.

So if you're trying to buy $2,000 worth of Vanguard Australian Shares Index ETF, which let's say is priced at $51.30 per share, then the most VAS you can purchase under $2,000 is 38 units worth ($1,949.40), with $50.60 left over.

This left over money will contribute to your next Autoinvest.