You can specify that custom sizes can be used for parts during layout, or you can allow only defined part sizes in the part catalog. For example, when creating a piping layout you might need an elbow with an angle that does not match any elbows in the part catalog. An elbow with the custom angle can be automatically created in your layout.
If you use custom sizes, any custom parts created are available only in the current drawing and are not saved to the part catalogs for future use. If your designs require custom sizes on a regular basis, see Creating Custom Content.