Verbal plenary inerrancy means that one believes all of the Bible is inspired down to the very words of Scripture. The belief in non-verbal plenary inerrancy would mean that one believes all the Bible is inspired, but only as to its concepts—not all the words—meaning that it might contain historical errors.
Charles Ryrie in his Basic Theology has this to say about the idea of “concept inspiration”:
Some are willing to acknowledge that the concepts of the Bible are inspired but not the words. Supposedly this allows for an authoritative conceptual message to have been given, but using words that can in some instances be erroneous. The obvious fallacy in this view is this: how are concepts expressed? Through words. Change the words and you have changed the concepts. You cannot separate the two. In order for concepts to be inspired, it is imperative that the words that express them be also. Some seem to embrace concept inspiration as a reaction against the dictation caricature of verbal inspiration. To them if inspiration extends to the words, then God must have dictated those words. In order to avoid that conclusion they embrace the idea that God inspired only the concepts; the writers chose the words, and not necessarily always accurately. But God’s intended concepts somehow came through to us unscathed.
Regarding the issues of inspiration, the following also from Ryrie’s Basic Theology may be helpful:
While many theological viewpoints would be willing to say the Bible is inspired, one finds little uniformity as to what is meant by inspiration. Some focus it on the writers; others, on the writings; still others, on the readers. Some relate it to the general message of the Bible; others, to the thoughts; still others, to the words. Some include inerrancy; many don’t.
These differences call for precision in stating the biblical doctrine. Formerly all that was necessary to affirm one’s belief in full inspiration was the statement, “I believe in the inspiration of the Bible.” But when some did not extend inspiration to the words of the text it became necessary to say, “I believe in the verbal inspiration of the Bible.” To counter the teaching that not all parts of the Bible were inspired, one had to say, “I believe in the verbal, plenary inspiration of the Bible.” Then because some did not want to ascribe total accuracy to the Bible, it was necessary to say, “I believe in the verbal, plenary, infallible, inerrant inspiration of the Bible.” But then “infallible” and “inerrant” began to be limited to matters of faith only rather than also embracing all that the Bible records (including historical facts, genealogies, accounts of Creation, etc.), so it became necessary to add the concept of “unlimited inerrancy.” Each addition to the basic statement arose because of an erroneous teaching.