What does it mean that BTC and BCH are not 'compatible' with each other?

I've heard that BTC and BCH are not 'compatible' with each other in SYS coin telegrams. What does "compatibility" mean here? Someone said it means BCH could not be sent to the BCH address, and BCH could not be sent to the BTC address. However, ETH can be sent to ETC, and ETC can be sent to ETH.

submitted by /u/shryu1220
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source https://www.reddit.com/r/btc/comments/8v1kom/what_does_it_mean_that_btc_and_bch_are_not/