There are situations when a partial diaphragms are required to be modeled. This caters to the situation when a diaphragm extends over only a portion of a floor. In addition, you may have to consider a situation when a floor is composed of a rigid portion (like- thick slab) and some openings or a relatively much flexible portion (like- corrugated steel decking). There may be situations when a high-rise have multiple towers where the floors are situated at the same Y-level. Such conditions demand multiple diaphragm definition at the same Y level of floors.
You need to select the nodes where the Rigid Diaphragm condition exists and assign the Floor Diaphragm to those nodes. You can have a master node created at the location of center of mass calculated by the program or defined by yourself.
If you have a situation where you have a rigid floor portion and relatively flexible portion, you can slightly move the nodes of the rigid portion in upward direction considering the tolerance set in the program. You can define a rigid diaphragm at the location of the nodes. The other joints which is the flexible portion of the floor would not be included in the newly formed diaphragm as they will be at the different Y level.