This article is from the storage FAQ part1, by Rodney D. Van Meter with numerous contributions by others.
19mm is 3/4 inch helical scan tape. Two varieties exist, D1 and D2. Both originated from broadcast and/or data recorder applications, where the data/signal was analog in nature. They have been modified for digital use, with error correcting capabilities added. Data rates are in the 8-45 MByte/sec range, with storage capacities in the 25-175 GB range in physically different size cartridges with different length tapes, but all fitting into the same tape drive unit.