I have a very similar table saw and the manual says that when you use the rip fence, the sliding panel has to be locked in place. Is there a reason for this?
I've never used a European-style table saw like that, so your mileage (kilometerage?) may vary, but in general with table saws you use either the rip fence or the miter gauge/sliding table, never both at the same time. The idea is that as you make a cut, one part of the workpiece (generally the one you want to keep) is constrained by the guide (again, the rip fence or miter gauge or sliding table), and the part on the other side of the blade should be free to fall away from the blade as soon as the cut finishes.
For example, if you're using the sliding table, the offcut will naturally move away from the blade slightly as the cut finishes, but it won't be able to do that if the rip fence is supporting it, and it's much more likely that a kickback will happen in that case.
The same idea is even more important when crosscutting. Let's say you're trying to cut several 100 mm blocks off the end of a long board. It seems like you could set the rip fence at 100 mm, set the board against the miter fence, and rapidly cut blocks by making a cut, sliding board over to hit the rip fence, making another cut, and so on. The problem is that those blocks are trapped between the blade and rip fence, and they could easily become projectiles if they happen to catch on the blade.
What you can do, though, is to clamp a small block of wood against the rip fence near the front of the table, and then set the fence so that the left side of that block is 100 mm from the cut line. Now it's fairly safe to cut the blocks as described above, using the block clamped to the fence as the stop block to position the board before each cut. In this arrangement, the fence isn't right up against the blocks as they're cut, so there's nothing keeping the blocks right next to the spinning blade.
However, when making rip cuts, it is quite tedious to align the board perfectly parallel to the disc. I was wondering if I can use the rip fence to align the piece.
If you're making a rip cut (i.e. cutting along the length of a board), skip the sliding table and use the rip fence -- that's what it's for. The rip fence is obviously long, and it provides lots of support for the side of the workpiece as you slide it along. The sliding table is for crosscuts, where the long side of the board is perpendicular to the blade and parallel to the miter fence (the thing that you've got marked with a question mark).
