Atransaction is when you "buy" or "sell" a particular asset of certain asset type ("stock" or "bond"). for instance, you can decide to buy 5 shares of vodafone stock which, given the portfolio described above, would result in you having 15 shares of vodafone stock. correspondingly, you decide to sell 5 shares of microsoft bonds, which would result in 10 shares of microsoft bonds in the above portfolio github