When I read, for example, 8 air changes per hour I get the impression that I must replace the room 8 times in one hour with FRESH AIR. Is it FRESH AIR?
Not unless it specifically says it is Outdoor Air Changes per Hour or that the room is to be 100% exhausted. A hospital patient room for example is designed for minimum 6 ACPH with minimum 2 ACPH of outdoor air (introduced in the central AHU).
Also worth noting is when they split outside air and total air requirements (such as 2 ACH OA and 6 ACH total), it carries an assumption that the supply air stream contains a portion of recirculated air from that particular space. If the air entering that room is a mixture of OA and recirculated air not from the room, all entering air can be considered fresh with respect to the requirements of that room.