The ALLOW_COPY keyword, which is implemented for KEY TEXT properties only, enables you to copy the property value by highlighting it and selecting Copy from the context menu (mouse right-click). Without it, the property depiction would be read-only, the edit control would be disabled, therefore disallowing text selection and copying.
Example
DEFINITION My Definition { ... PROPERTY "Property" { KEY EDIT TEXT LENGTH 2000 ALLOW_COPY } ... }