Custody accounts are accounts held with Brokers (Security Portfolios) and/or Banks, to buy public securities (stocks, bonds, funds) or Cryptocurrencies (in which case they may also be called Exchange/Custodial Wallets).
We explain custody accounts because with Exirio you can choose to add your investments in securities and/or crypto as a single, manual investment. If you do so, your transactions and valuations tracking requires a little more explanation (here for securities, and here for crypto).
They are called “custody” accounts as the institutions that offer them hold in custody the investments you instruct them to buy, on your behalf.
Custody accounts typically consist of a cash and a securities component. In other words, if you transfer 50,000 USD into an account with XYZ Broker, at the beginning you will only hold cash. As you use this cash to buy securities, you will be increasing the securities component and reduce the cash component.