From Online Manual
Moderators can split Posts made in Topics. This means they can take Posts in a Topic out of that original Topic and have the selected Posts in the original Topic form a new Topic. To split Posts from a topic:
Go to the Topic with Posts that should be split. To the right of every Post, you'll find a button that says Split. Selecting the Split button next to a Post will direct you to a screen that asks you what type of split you want to make. There are three options you can choice from.
The first option: Only split this Post, will only take the Post that you clicked the split button next to out of the Topic and have that Post become its own newly created Topic.
The second option: Split Topic after and including this Post, will split the Post that you clicked the split button on and all of the Posts made after the selected Post and create a new Topic with those Posts.
The last option: Select Posts to split, will take you to a screen where you can select all of the Posts you want to split out of the Topic. On the Select Posts to split you will see two columns. The first one has all of the current Posts that are not currently selected to be split from the Topic. To split a Post, simply click on the arrow pointing towards the right column, and you will now see that Post on the right column where all of the Posts selected to be split from the Topic are placed. You can undo a Post you had previously selected to be split, by clicking on the arrow pointing to the left column, moving it back to the left column with the Posts currently not being split from the Topic are.
When you've selected the boxes of the Posts to be split, click on Split Topic. You'll then have the option to either go to the Message Index, return to the Original Topic, or visit the New Topic.