I'll start with a question: "Why the fifth hole?"
Probably I should "frame" it so that it makes more sense:
As reasonably-advanced techs, we still remember how we used to think that manually-controlled mono-block and sectional directional control valves were simple four-way three-position devices:
How silly we were back in the day, right? To be fair, a lot of beginner hydraulic courses do introduce manual DCVs to their students with the four-way symbol for simplicity's sake. But nonetheless - now we all know that a typical (mono-block or sectional) manually controlled DCV has a by-pass gallery that is closed by the by-pass-closing part of the spool and a pressure gallery that gets distributed to the work ports either via a common single, or multiple individual check valves.
How about that?! One gallery for "close-able bypassing", one gallery for "pressure distribution", and a tank line for, well, tanking (I guess). And so now, when we see a typical work section of a sectional DCV, we know why it has four galleries on its side - the central by-pass and the pressure in the middle, and a couple of tanks on the sides:
This is great, and is also a perfect segue to my question: "Then why do some sectional valves have an extra (fifth) hole in the center?" For example - this SDS150 here:
The side holes are definitely the tank holes, because the line relief valves vent directly into them, two of the holes in the center are supposed to be the by-pass and the pressure, but what about the bottom one? Weird? Not weird?
Now, obviously, if you already know the answer to this question - you're a top-tier tech who has worked with enough of these valves and studied enough catalogs, and I salute you, but if you don't know what the fifth hole is there for - you're about to find out.
It has all to do with the possible functions that a sectional valve is supposed to be able to provide, and the choices of the very intelligent people who designed it.
First of all, let us discard the side holes - which indeed are the tank lines, but only for the port valves (pressure relieves or anti-cavitation checks, plus they also connect to the spool side drain cavities) - but they have no function in the "oil distributing business", not in this design anyway. So... where did the tank line go then?
If you study the section's body, you'll see that the hole in the center is indeed the central by-pas (let's call it "C"), the top hole, which so obviously feeds into a check valve that is located between the work ports, would be our pressure gallery (let's call it "P"), and the third hole (the "mysterious" one) is the return line, and the fact that the very clever engineers thought of channeling the return flow into a separate gallery, gives this design the ability to combine not only the good old parallel operation where all of the functions are fed from a common pass-through gallery, and the return flows are dumped to the tank, but also the series and the tandem operation as well. And you don't need any special spools for that! (But you do need special bodies).
On a side note - I use the terms "series" and "tandem" because that's what you find in catalogs. To be honest - I find the terms to be misleading. In my head, tandem and series are kind of the same concept. I mean - any sectional valve is a tandem assembly, is it not? Be it as it may, I can never remember which one is which, and this is why I like making a "dumbed down" drawing, that makes the "head-wrapping" around the functions of sections easier. For the SDS150 it would be something like this:
Now I can look at the diagram and recapitulate.
Parallel configuration: The one you'll use most of the time, I would say 99.9% of the time, if not 100% of the time. The pressure and return galleries happily pass through the valve stack, and everybody gets to be fed from the same common pressure gallery. Whoever has the smaller pressure demand - wins!
Series configuration: When you activate such a section on its own, it is fed by the central gallery, but if you activate another section upstream of this one, the return from the upstream section will feed it now! Very useful for series motor operation (think multiple horizontal transport belts).
Tandem configuration: The word tandem still confuses the hell out of me, I should stare at the drawing for another minute... Since this one is fed exclusively from the central gallery, any active tandem section will inhibit the operation of the tandem sections downstream of it. I guess this can be useful in some situations. I can't recall the last I needed one of these, though.
You can also tell that you can combine various types of sections if you are "in a pinch". For example - both the series or the tandem section (of this design) can be the first section of any parallel stack.
Another useful thing about the drawing is the fact that you can easily identify the sections just by looking at them: both the P and the R are pass-through - this is a parallel section. The P and R are blocked? Must be the series section! Only the P is blocked, and the R is pass-through (see-through) - that's the tandem all right!
By the way - the four gallery design can also perfectly achieve the series and tandem functions, but with slight differences, especially when you need to combine different functions in one valve stack. My advice is - don't worry too much about it, in real life you'll most likely never need it, but you do need to know that these types of operation exist.
If you really want to wrap your head around the possible functions of the sectional DCV of your company's first choice, study the respective catalog and make a simplified drawing as I did in the example above - it does help make things a lot clearer - believe me!