The reason is that we can trade several strategies on same symbol (different charts) with same Expert_Magic in order to achieve more complex logic.
We can have several strategies that make entries at different levels during the bar and other strategies that hes different exit levels.

That is if the FST ID number for the MT4-FST_Expert attached to a EURUSD 15m chart was 55, the MagicNumber was to made 55 also.

Not necessary. Leave Magic as it is by default.

Also it is not necessary.

Just leave it default. You don't need to change it in most of the cases.

Expert_Magic must be changed ONLY if you run strategies on same symbol.
Examples: 5 strategies on: EURUSD M15, USDJPY H1, USDCHF 1H,  EURUSD H1, GBPUSD H1.
Here you HAVE TO change the Expert_Magic for EURUSD M15 only, in order to separate it from  EURUSD H1. For all other strategies you can leave the default Expert_Magic. (or to change the magic number for  EURUSD H1)

Setting different magic for each strategy running on different symbols is not wrong but is useless.

FST Expert separates deals by the symbol. Expert_Magic serves to separate deals if more than one strategy runs on same symbol.

If you have two strategies on EURUSD H1 and USDCHF H1, FST Expert knows that positions opened by the first strategy are for EURUSD and the second strategy opened positions for USDCHF. So here we do not need to change the magic.
But if we have strategies running on EURUSD H1 and EURUSD H4. FST cannot separate the positions of the strategies only by their symbol. Here comes the Expert_Magic. We have to set different magic for each strategy since the symbols are equal.

For Connection_ID is clear. Each expert has unique ID number.

You need to change Expert_Magic only if you trade different strategies on same symbol. Meta Trader Experts use Expert_Magic for separating its deals from other experts. This number is something like ID number for the Expert Advisor.
Forex Strategy Trader (the program) doesn't use the Expert_Magic number. It is known and used only from MT4-FST Expert.
FST uses Connection_ID number to separate information feeds coming from the different running experts. That's why we have to set different Connection_ID number for each expert. In the other case, the information coming from different Experts with same ID numbers will be mixed and FST trades will be wrong.

Thanks a lot for Your help

it seems that everything is ok.

You wrote:

1. Open 3 charts, put expert on each, but you must assign unique ConnectionID & MagicNumber on each.

MagicNumber ? What do You mean ? "Expert_Magic" variable ?

What for is this ?

Thanks for support

Run multiple is same thing:
1. Open 3 charts, put expert on each, but you must assign unique ConnectionID & MagicNumber on each.
2. Than you open 3 FST, and you need do configure each with specific ConnectionID (it can be enabled from options - Allow multiple..) , you provide in point 1

Thats all.

because Youtube Tutorial about "How to configure multiple sessions of FST" is restricted in Germany (Music Owner Rights) I would like to ask You about small tips.

How may I run multiple sessions of Forex Strategy Builder ?

On my PC I have installed two MT4 in separate folders. Each installation is for different Live account.

c:\Program files\MT4-1
c:\Program files\MT4-2

For each meta trader I would like to run 3 different strategies:1) EurUsd 5min 2) GbpUsd 5min 3) EurUsd 1h

How to connect each strategy with dedicated chart in Meta Trader ?

Would be great if somebody could help me.

Dlugi