This is still an experimental feature and might change visually (icons, names) or in behavior. You need to turn it on in the preferences under the "Lab" options (on Mac & iOS, the setting is synced between devices):
What are Synced Lines?
A Synced Line is a single line of text, such as a task, quote, bullet point, etc. that is mirrored across notes. If you paste that Synced Line into another note and change the content of that line of text in any of the notes, NotePlan will sync this change to all connected copies.
Using Synced Lines you can for example create a task inside a project note and schedule it to a daily note. Inside the daily note drag the task from the references into the note and press CMD on your keyboard when dropping it (on Mac, on iOS use the asterisk icon in the toolbar inside the original note). Now you can move the task under a new heading or into a list. When you check the task off, it will be synced back to the original project note and everything stays up to date.
How to create a Synced Line
You can create a Synced Line either by dragging a reference into the note and keeping the CMD key pressed on your keyboard when dropping (Mac & iPad) or (on Mac) by clicking on the drag button which appears when you hover over a line of text to open a context menu, then select "Copy Synced Line". On iPhone you can press the asterisk icon in the toolbar after putting the cursor into the task you want to sync.
On iOS, you can tap on the asterisk (*) icon in the toolbar after selecting the text you want to copy as a Synced Line.
Once you have copied the Synced Line you can open another note and paste it anywhere.
How it works
When you create a Synced Line, NotePlan attaches a unique ID to the line of text like
^129abz and uses this ID to find all copies of that line across notes when an update is due. The ID is displayed as an asterisk icon in the text.
Navigate between copies
You can find out where other copies of a line are saved by clicking on the asterisk symbol. A popup will open with a list of notes.