Niederrlassungserlaubnis (NE) is the name of the visa given for immigrants who apply for Permanent settlement/Residentship in Germany. Another variant of this is the EU wide Permanent residentship called  Daueraufenthalt EG.

The actual requirements of a Niederlassungserlaubnis (the word itself is a long one :( ) are in general:

• Integrationkurs (for those who came after 2005)
• Proof of financial support (Payslips etc)
• Job Contract
• Proof of Pension payments for 60 months excluding unemployement
  
• Proof of university degree if studied in Germany
• Health Insurance Certificate
• Job Contract and other proof documents as stated by the authorities.

The documents required difers from officer to officer who is inspecting the case. And its in your best interests to fulfill the requriements. Its also handy to know the laws and rule numbers in case they contradict.

Asking in return is always helpful to get a reply based on research of options.

The requirements in general are same for NE and DG. With DG , one can work anywhere in the EU but then NE for Germany is void(Erlischt). The main thing is that who ever finishes 60 months of fulltime employment paying social contributions are eligible. However if you happen to be a student before, they consider half the time of student period.

To summarize:
NE      = 60 months of stay with social contributions
NE/DG = (Number of months as student / 2 ) + Months of employment = 60

NE for dependents and other highly skilled scientists is out of scope of this piece of information. This is also not an official advisory. Please refrain from claiming any sort of anything.