its 50% of the total nation army.
Lets say you have 10 million infantry and 10 million infantry in Nation Army (NA) with 50% Nation army dispatched (the maximum you can have) and you want to attack someone (lets say Standard Attack(SA)) and tick the "use national army" box, then your overall attack power in the SA will be 15million infantry. Btw nowadays people play in small nations (most solo) and they rarely have full NA, especially if they are not playing indy (only using industrial zones and construction sites)