Slide 13 of 41
Notes:
If every encoding has a T part, what is the numerical code for each type to go into that T part?
In ASN.1 these IDs are called tags, and are default values are assigned to every ASN.1 primitive type (and to the construction mechanisms SEQUENCE and SET).
If a field in the encoding is (for example) a single TLV representing a value of:
then the default tag for the BOOLEAN and the INTEGER type can be used in the encoding, and we have no problem.
If however, as is illustrated in the slide, the CHOICE involves types which have the same default tag then use of that tag in both cases would produce ambiguous encodings.
The case of INTEGER and NULL is illustrated, but a more common case would be where both alternatives are SEQUENCE types.
