if a country wants to field 2 or 3 athletes in an event all of them should have met the A standard in the qualifying period. in order to field only one athlete in a particular event that athlete must meet the B standard. if a country does not have any athletes that meet any of the standards they are allowed to have one male and one female athlete so all nations could be represented. i think the home country can field at least one athlete in every event if they choose to do so.
Also some countries have their own standards which are more difficult than the iaaf's to ensure that the athletes they send will not go to the worlds as tourists, that they are going to be competitive.