For Market by Order (MBO), the price direction in the K tag is set by comparing the latest execution price (or, if there is no execution, the base price at the start of the morning session). For example, the opening price of the afternoon session is compared to the last execution price of the morning session. This specification is designed for use as the uptick/downtick indicator for Short Selling Regulation.
On the other hand, for Standard realtime distribution, the determination is generally made per session. The change flag in the 4P tag Current section is set by comparing the latest execution price within the session, and if there is no execution price to compare, "4" (other than "Going Up", "Same", "Going Down") is set. For example, the opening price of the morning and afternoon session is always set to "4".