no, it means if you cant have pokemon for that battle,
say the max pokemon is 4. u have 2 pokemon at that time, but u are 2 short,
u can catch 2 pokemon and level them up(if u want), those 2 wont bein ur team, if u dont want them to, so they are substitutes :P
also is either tetiro or tman availible for a heartgold challenge or platinum one, its ok if ur not, its just that i cant get wi-fi with gen 4 games