Additional arbitrary attributes are allowed within each feature definition. Default feature attributes Default values may be specified for any attribute-value pair. Any attribute defined in the default definition will be included in all other feature definitions, unless that particular definition specifies a different value for the same attribute. The following two sets of feature definitions, A and B, are functionally equivalent. Introns maintain frame without changing it, so the frames of all bases within an intron are equal to the frame of the final base of the preceding exon.
Passive signals maintain frame, just as passive contents do.
Since all signals have zero length, only contents require a length specification. It is legal to define a fixed-length content without specifying what the length is. In such situations, the learner is obliged to choose the content length. Similarly, the inclusion of minimum and maximum lengths for a variable-length content is optional. Without them, the minimum is zero and the maximum length is infinite.