Customizing (by account) means that you can create and edit different content for your accounts in one single post. Customization is available when at least two accounts are selected. Currently, customization is not available for stories.
How to customize
Create a new post or open an existing post to edit. Now you can type text and add images. This content is copied automatically to all social accounts selected for your post. Start customizing by clicking the By account button at the bottom right corner. Note, that Smart content suggestion is not available when you customize posts.
Modify the content of the post separately for any account by swiping right or left. You can modify, add, or remove text and change pictures per social account as you wish.
You can also select accounts you want to post your message to by clicking the round Add account in the top left. Note that, you can currently select only one Twitter account at a time. Twitter won't let third-party apps post to multiple Twitter accounts simultaneously. Also, you need to verify your Googe Business Profile before you can select and post to it.
You might get a "Customized content!" warning if there is customized content in your post. This is just to remind you that you don't accidentally overwrite the account-specific content of the posts.