It's a little disappointing to see Digitec, the specialists for all things digital for many people, putting some RAM DIMMs with 2400 MHz and 2666 MHz frequencies into the matching accessories of the Ryzen 5900X. I guess it's matching in the sense that it will work but you won't get optimal performances out of it. When spending more than 500 bucks on a processor you should get the best tips and information from your hardware provider (looking at you Digitec). DDR4 stands for double data rate of the 4th generation. It means that a memory at 3600Mhz really runs at 1800Mhz (or cycles per seconds) but sends information twice per cycle. The 5900X has a fabric clock of 1800 Mhz as most Ryzen processors do. (not to be confused with the core speed in GHz). These processors work best when paired 1:1 with the RAM (otherwise they have to wait on each other regularly and some clock cycles will be "lost"). Meaning that if you get a Ryzen processor you should aim for DDR4 DIMMs at 3600 MHz for best performances. (4000 MHz if you plan on manually overclocking your CPU fabric clock to 2000 MHz).
Is it an absolute must? No. Will you notice the performance decrease? If you're not a rendering artist, a CAD engineer or a gamer, chances are you won't. But then it should be your decision to save a buck and downgrade your memory and not have it be the consequence of you not getting the right information.
If you're not after performance though, you might want to reconsider your choice of a super high end CPU like the 5900X all together.
Digitec is a great shop. It became very (very) dominant in Switzerland. I just hope it won't become complacent. Good tips to customers and attention to detail is crucial when selling computer hardware. The specification pages of many items have missing, inaccurate or wrong information (see the confusion in memory speed vs memory bandwidth in RTX 3080 graphics cards for instance, if it hasn't been fixed). Don't recommend sub optimal RAM because you want to move product. It's not a nice color...
@Anonymous no worries with the Dominator 3200MHz. You can easily overclock this Memorykit to 3600 MHz without touching the timings. Just hop into the Bios and set 3600MHz manually and you're good to go (adjust voltage as well to like 1.35V). I have the same Memorykit and overclocked it to 3600MHz and even tigthen the primary timings + subtimings and it's absolutly stable.