Do you mean which ideas can be approved and which not?
If so, this is my say on it.
If an idea that is being filed falls under any of these, will have tough time to go through and be approved as a patent.
1) Abstract Idea - An idea even though being unique but in reality difficult to implement or not specific is hard to get through.
2) Computerizing a "Business Method" already in practice in its convetional form cannot be considered as an invention.
3) Automation of an existing activity or mental process cannot be considered as an invention.
For example - Verifying the validity of credit card transactions over the Internet, and the steps in the method could be performed in the human mind or by a human using a pen and paper.
4) If an idea can be mapped to an existing process then that idea cannot be considered as an invention.
For example - Collecting data from hard copy documents, processing it and storing the information in memory is not an invention as it maps to an existing scanning and processing technology.
5) Mathematical algorithms which merely manipulates existing information to generate additional information is not patent eligible.
You can refer to https://en.wikipedia.org/wiki/Software_patents_under_United_States_patent_law for more details on all of the above.