Capabilities exist in many varieties but all have a Default Value, Current Value, and may have other values available that can be supported if selected. To help categorize the supported values into clear structures, TWAIN defines four types of containers for
capabilities: TW_ONEVALUE, TW_ARRAY, TW_RANGE and TW_ENUMERATION.
| TW_ONEVALUE | A single value whose current and default values are coincident. The range of available values for this type of capability is simply this single value. For example, a capability that indicates the presence of a document feeder could be of this type. |
| TW_ARRAY | A rectangular array of values that describe a logical item. It is similar to the TW_ONEVALUE because the current and default values are the same and there are no other values to select from. For example, a list of the names, such as the supported capabilities list returned by the CAP_SUPPORTEDCAPS capability, would use this type of container. |
| TW_RANGE | Many capabilities allow users to select their current value from a range of regularly spaced values. The capability can specify the minimum and maximum acceptable values and the incremental step size between the values. For example, resolution might be supported from 100 to 600 in steps of 50 (100, 150, 200, ..., 550, 600). |
| TW_ENUMERATION | This is the most general type because it defines a list of values from which the Current Value can be chosen. The values do not progress uniformly through a range and there is not a consistent step size between the values. For example, if a Source's resolution options do not occur in even step sizes then an enumeration would be used (for example, 150, 400, and 600). |
CapType property