Hi Anton,
The Phase is the subprocess that will split each lane horizontally or vertically based on the swimlane orientation. If you want to add children for swimlane you have to add the children to lanes only based on the LaneOffSetX.
For example, if you create a swimlane with 3 phases. Each phase has a width of 200 then if you want to place children in the 2nd phase you have to set its LaneOffsetX value between 201 to 400.
For more details about Swimlane, please find the below UG link.
Regards,
Karkuvel Rajan S