A Deposit Transaction occurs when you make a new Investment, or when you put more money into an existing Investment.
If you buy a house for 200,000 USD, on Exirio you add it as a new Investment and a Deposit for that amount is automatically created. If you add $50,000 to an existing Broker Account, you have deposited that amount in the Broker Account Investment. The important thing to recognise with a Deposit is that the value of your Investment increases (the value of your Broker Account will be $50,000 larger after you have made the Deposit).
On Exirio, we use the term Buy instead of Deposit every time you are working on Security Portfolios, Crypto or Precious Metals: but the meaning is exactly the same.
On the graphs, changes in value caused by a Deposit (or Buy) are marked with a D (or B).