Though many promos are intended to run in an exact position in an episode (i.e. the second Mid-Roll spot), promos can also be set to run as positionless. When "No Position" is selected, a promo will insert into any open position within the selected unit type (Pre-, Mid-, or Post-Roll) wherever available.
To set up a positionless promo, start by Creating a Promo (or selecting an existing promo).
Under the Position section, mark "No Position".
Doing so will allow the promo to run in any open position for a designated unit type. For example, if the promo has been selected as a Mid-Roll location with no position, Megaphone will insert the promo into any open Mid-Roll spot (be it the first Mid-Roll, second Mid-Roll, etc).
In order for the ad-insertion to behave as described, be sure to have ad locations defined in order for the promo to run!