Open architecture systems may use a standardized system bus such as S-100, PCI or ISA or they may incorporate a proprietary bus standard such as that used on the Apple II, with up to a dozen slots that allow multiple hardware manufacturers to produce add-ons, and for the user to freely install them.
By contrast, closed architectures, if they are expandable at all, have one or two "expansion ports" using a proprietary connector design that may require a license fee from the manufacturer, or enhancements may only be installable by technicians with specialized tools or training.
Computer platforms may include systems with both open and closed architectures.
A second type of open software architecture consists of the messages that can flow between computer systems.
These messages have a standard structure that can be modified or extended per agreements between the computer systems.