Basically both plug-in instances are identical but current Acqua engine can work with long audio buffer or without any audio buffer.
The instance without audio buffers, or "ZL", or zero latency, do not have any audio buffer pre-loaded, and will process the audio without any delay, so at the same time CPU load will be higher compared with standard non ZL instance.
The idea behind a ZL instance is give users the option to run Acqua Effect products with a minimal latency, useful for direct monitoring for example.
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