Income is the Transaction that takes place when you receive money in relation to an Investment, but without any corresponding change in value. For example, when the tenant in your property pays you the rent, you receive that cash but the apartment does not change in value because of it. Or when you receive a coupon from a bond, or a dividend from a stock and you move the money out of your Security Portfolio.
There is one exception to this rule: with Crypto and Savings Accounts & Deposits, the value of the Holding grows by the value of the Income received. For example, when receiving a $1 interest on a $100 savings account, the balance (i.e. the value) of the account will grow to $101.