To join segments, just click on a segment, and then click the Join button. The segment below will be joined to the segment in which you have your cursor. The blue "j" tag will appear, signaling the segments are joined. You need to keep this tag in the target segments as well as in the source.
Please note that the "j" tag in the target should be inserted into the same position as it is inserted in the source when the segments are joined. This tag effectively marks where the segments were joined. If placed incorrectly, it can break the formatting of the document or the document itself.
To split a segment, position your cursor in the source section at the spot where you want the segment to be split. Then click the Split button.