An operative is In Cover when two conditions are met: a terrain is Intervening, and any intervening part of the terrain is within Control Range of the operative. In simple terms: the intervening terrain must be close to the target to work as cover. Being in cover gives the defender a **cover save**: they can retain one die as a normal success without rolling it.
If the terrain does not cross the base-to-base lines, it is not Intervening, and the target is not in cover. Remember, it's the attacker who chooses which point on the operative's base to use as the origin for the targeting lines. In the scene below, the defender is not in cover because the attacker decided to draw targeting lines from the left edge of the base.
Compare different scenarios of being in cover. Note that it's enough for the terrain to intervene with any part for it to work as cover. It doesn't need to block the intervening lines completely.
An operative cannot be In Cover while within 2" of the other operative. Here, the target has Intervening terrain within 1", but the attacker is too close.