The SRD is the one coat product. This is a semi transparent stain, so the wood underneath the stain is meant to show through. The DEK product is a solid stain that will behave and apply somewhat like a paint. This product can be applied in multiple coats.

We sell LOTS of Sikkens where I work!
I've been around the stuff for over a decade, & helping people with their decks.

Since you didn't specify which type of Sikkens exactly, I'll start with the most probable:
>>>>> SRD- This is the Semi-transparent (Oil OR Water-based...both types available).
* You'll see all the wood-grain here.
* SRD is a PENETRATING stain....NOT a film-forming product.
(think of rubbed-in sunscreen here, vs. applied makeup, which largely sits ON TOP of your skin!)
* Too much SRD applied, OR....too many coats, will lead to a dried "skin" on some or all of the deck. This will behave like a blistering/peeling sunburn.
* If 2 coats of SRD "have to be" applied...WIPE-OFF whatever doesn't penetrate in 10-15 minutes...NO LONGER.
* Whatever doesn't soak in during that 2nd-coat time-frame WON'T....and WILL lead to peeling.
* NO MORE than 2 coats of this!!!

>>>>> DEK-Finish:
* This IS a Film-forming product!
* TWO COATS MANDATORY on a "new-wood" scenario.
* Plus...the 2 coats MUST be applied to ALL SIX SIDES. This keeps most moisture out of the wood in the first place, which could lead to peeling of the "topside" coats.
(think of the outer crust-of-bread idea)
* Since most people aren't gonna crawl under a deck that's built, this product isn't used much!
* On furniture, handrails, planters, benches, etc., it looks GORGEOUS!
* It's literally TWICE the price of SRD too. Our DEK-Finish is ~ $90/gal.

>>>>> Log & Siding or Cetol 1/23+:
* I hope He's not talking about these, since they're VERTICAL surface ONLY products!!
* These also form a film.

Faron