On multiple 485s

Not sure about other situation, but below:

"Pending derivative I-485

An individual with a pending derivative I-485 can file a second I-485 as a principal alien. The second filing will be consolidated into the same A-file. Applicants receive only one A-file number. Another way to filing a second I-485 is to a copy of the I-140,I-130 or I-360 approval with any documentation that is required for a primary applicant抯 I- 485 like an employment letter. In this case it is important to take priority dates into account. "

I found this on the web but cannot trace the authorative source. Any thought?