I/O attributes are the characteristics of logic macros or nets in your design. They indicate placement, implementation, naming, directionality, and other characteristics. This information is used by the design implementation software during the place-and-route of a design.
Input and output attributes are described in the documentation for the I/O Attribute Editor. Attributes applicable to a specific tool are described in the documentation for that tool.
See the topics in I/O Attributes Reference for more detailed information about each attribute. See also I/O Attributes by Family, for a table of attributes for each device family, and Welcome to I/O Attribute Editor.