It is sample accurate if you use it in a frame container. Otherwise it is the ModulationBlockSize that is taken but I am not 100% sure how. Because if an audio block is 512 and the modSize is 16, I don't see how it works...
If the ModulationBlockSize is set, then HISE will split up the incoming buffers and send out parameter values with this control rate, so:
512 samples in, ModulationBlockSize = 16
- calculate all modulation values and send the parameter callbacks
- process 16 samples
- calculate all modulation values and send the parameter callbacks
- process 16 samples
...
Note that the frame container will not make this sample accurate because that's a inner property of the network and HISE has no idea whether you use frame processing from the outside.
That's why it's recommended for high-resolution modulation requirements to use the extra_mod nodes and connect it to the slots - they can fetch the modulation signal from within the node and apply it to the signal (using multiplication) or modulate any parameter with the resolution you want within your network.