There are a relative few scout bees that make the decision of What, Where, and When, to work a particular flower source. and whether to work it for nectar or pollen. the scouts come back into the hive, give out free samples and through the dance tells the field force bees where it is. the more abundance A particular flower source is, the more scouts will be presenting it, getting more bees to go gather from it. Also the bees given a choice of different nectar's from different scout bees will choose to go after the sweeter nectar's most of the time. But quantity of nectar the plants produce, and the distance (flight time) are also factored into which nectar sources are being worked.
A forager bee comes back, passes his nectar to a house bee, could be offered a sample of new nectar from a scout bee that has just returned to the hive. Noe the forager bee has a choice, keep working what he has been or switch to the new source. That decision is based on whether the sugar content is higher than the source they have been working, The distance needed to travel The volume of nectar will effect the time needed to gather, and how many flower need to be visited during a trip.