Why call it “EXPLICIT” tagging?
Because it is the opposite of “IMPLICIT” tagging!
If the original (default) tag is still there, then the type is still explicitly identified and the added tag does no harm (except for extra bits-on-the-line!)
In modern ASN.1 specifications, the user can ignore tagging issues with something called AUTOMATIC TAGS
Notes:
The term “explicit tagging” was really introduced because there are just the two sorts of tagging, and in English the opposite of “implicit” is usually considered to be “explicit”.
However, the name is still about right, as the old (default) tag is still present, and so the type of the V part is still explicitly identified without any knowledge of the type definition.
In specifications written to the ASN.1 standards from 1994 onwards, it is possible to include at the head of the specification AUTOMATIC TAGS. In this case, all replacement or addition of tags is done by an algorithm (essentially as a textual transformation prior to encoding), and designers do not need to understand or to use the tag concepts and notation.
Most designers today, however, still seem to prefer to take control of tags themselves, particularly when BER is being used. When PER is in use (because the actual values of tags are unimportant), it is more common to see a designer choose to switch on automatic tagging.