BitCryptex, nc50lc thanks a lot! I got it now. So I think it would be better to generate seed not in Electrum, but in other BIP39 services and then use Electrum.
no it won't be better. Electrum is an open source software, in case some day in the future you couldn't run Electrum anymore and needed to convert your seed into an extended private key (xprv) to import in another wallet, it would be so easy to do so since the method is already known.
not to mention that what Electrum does under the hood is safe but what you do yourself with another tool may be considered unsafe because you may not be aware of what you are doing exactly.
BitMaxz, BitCryptex thanks for the advises! I'll do this way. Which standart will you advise BIP32 or BIP44?
BIP44 is not a different standard. it is a suggestion for the way you can use the "path" in BIP32 so if anything it is like an extension to BIP32. so is BIP39 which is a way to store your BIP32 entropy in form of a seed phrase. at the end of the day you are still using BIP32.