These two options allow you to:
* For testing on a single or a small set of graphs, specify those graph ids
* If you only with to update one of the aggregation levels, say for example daily ~ 86400 step size, do that or greater. This means that if you have the 'default' cacti which is 5 minutes, 30 minutes, 2 hours, and 1 Day, you can limit which steps or greater to update. This is good for updating RRDfiles where the data occurred weeks, months or years in the past.
TheWitness