That is a group. In field events, when there are a lot of participants, they will split them into flights. For example, if there are 20 girls in long jump, they could split them into 2 flights of 10....
A 'heat' is a preliminary race in a running event where the the top 2 or 3 finishers qualify for another race and the others are eliminated. For example, if there were 32 entrants in a 100 meter dash...