Reaper supports both Host and CC/MIDI automation for sending information from Reaper to control parameters in a plugin, for example. Below, I will provide a few examples of using Kontakt Player as a plugin. In Kontakt I’ve loaded the drum instrument Tympo.

CC/MIDI automation

For example let’s map CC 12 (just a random number, you can choose your own) to the Tune parameter of the Hihats. Here’s how that looks when we map it in Kontakt Player:

In Reaper in the left bottom corner we can now select CC 12 and draw some automation like this:

This will dynamically tune the Hihats according to the automation lines.

Host automation

Now we want to map # 0012 (just a random number, you can choose your own) to the Tune parameter of the Hihats. Here’s how that looks in Kontakt Player:

To automate it we need to select the parameter via the Track envelope (choose Param in the plugin and select Show track envelope):

Now we can draw the automation any way we like and it will look something like this (I am using a custom theme so it might look a little different on your system):

The result is exactly the same as the above example using CC/MIDI: the tuning of the Hihats is automated.

You can move this automation to the media lane if you want:

Now the automation will be displayed superimposed on the MIDI item:

What method is best?

Both techniques, Host and CC/MIDI, will produce the same results. The only difference is that they appear in different windows/UX-elements.

When programming MIDI parts, you might prefer to use CC/MIDI automation for controlling the sound because it is displayed directly beneath the MIDI events/notes.

Easy peasy, right? Good luck!

UPDATE: user mschnell added some additional information on the Reaper forum:

CCs are supposed to be handled sample accurate with all plugins.

Parameter automation can’t be handled sample accurate by VST2 (by definition of the VST2 API) it can be handle sample accurate by VST3, but AFAIK, most VST3s don’t do that.

Reaper forum

Nice to know (sample accuracy when using CC)!