Interval
The Interval configuration option is a top-level configuration option in the collectd config file. It controls how often registered read functions are called and with that the resolution of the collected data. By default an interval of 10 seconds is used.
Internally, the interval is stored in the high-resolution-time-format. The configured default resolution is available from a global variable called interval_g.
When registering a read-callback, a plugin may specify an interval in which to call the function. Since the majority of plugins does not use this feature, this global default controls very many statistics.
When sending values to another instance using the Network plugin, the interval for each value is included in the network-protocol. Therefore, client(s) and server(s) do not need to have the same default interval configured.
The files created by the RRDtool plugin heavily depend on the interval of the value they are supposed to store. If you change the interval of a value, you have to delete the corresponding RRD file. If you change the default interval of an instance, it's probably easiest to delete all RRD files belonging to that host.
Scripts executed by the Exec plugin can dispatch values in arbitrary intervals. All they have to do is set the interval option of the PUTVAL command correctly. To use the default interval in those scripts, too, you can use the COLLECTD_INTERVAL environment variable (since version-4.9).