Poloniex customers can choose whether or not to split their BCHABC into BCHA and BCHN before the upcoming November 2020 fork. Customers that would like to split their BCHABC before the fork can do so from within their Wallet.
The criteria for splitting and merging BCHABC are as follows:
1 BCHABC = 1 BCHA + 1 BCHN
Merging BCHA and BCHN
1 BCHA + 1 BCHN = 1 BCHABC
Similarly, users that hold BCHA and BCHN (whether by splitting BCHABC or by buying it in the pre-fork markets) will be able to merge their pre-fork assets into BCHABC from their Poloniex Wallet.