Some questions about programming Uniwell UX 40

I don't know anything.. I've purchased the simple programming guide, but I'm afraid I'm getting stuck on the simple things. I don't know the basic stuff, or perhaps sometimes I just don't know what that particular word means.. Not getting anywhere. I hope someone or everyone can help

First: What's meant by the "PLU"? Is it just a code that you assign to a particular product/article? And what are those HALO things?
I want to make some groups. So that I first type in the amount, then type the key assigned to this group so it gets registered. The groupname should then also get printed on to the ticket. All of the groups have the same VAT %, except on, so I have to assign a certain VAT % to certain groups.
In the manual it says: enter the group number of the group you want to change. But what is the group number?????
I suppose it's main groups I have to make because there aren't any subgroups?
To get into my business: I've got an aquariumshop. I want to make groups like: aquarium, heater, pump, fish, plants, ... Only groups, but no specific article with specific prices.

That's the first thing I'm sweating on, and someting which will be fairly easy solvable I think, once I get the basics.. But the other thing I want to ask is more difficult.

My cash register is old. It still got our old currency (BEF), and euro. I want it to just have euro. The TTL5/cash button is set as BEF. So everything on the ticket says BEF. When I press the euro button, which they have programmed on the TTL4, it immediately ends the 'transaction', as if the client only wanted 1 article. When I press the subtotal button, it doesn't give me the subtotal, but it changes from one second to another to euro and BEF. That's the problem. So what I would want to do is this: I want BEF (the name) to be changed in EUR. I want to delete the programming on the TTL4, so euro is no longer on it, and it's empty. I want the subtotal to give me the subtotal, and not the amount in BEf and euro.
Since this cash register appears to being used not only in Belgium, I suppose it has to be possible to change the name of the currency.. And to delete the euro currency thing.

I know it's a lot to ask for, but I do so hope that somebody can help me. Otherwise I've made a baaaaaaad purchase...