A customer requested advanced NSQL construct examples some time ago and I promised to follow up. Here is what I was able to find out and validate with our CA Support team:
1.) @INVESTMENT_SQL@ - obsolete. The @INVESTMENT_SQL@ construct is no longer valid for 13.2 and beyond; it was primarily used for the pre-13.2 portfolio portlets.
2.) @NVL@ - check for null value (similar to ISNULL database SQL function)
3.) @+@ - concatenation of fields in a SELECT list
4.) @SYSDATE@ - database system date
As for the other constructs numbered 2 to 4, examples of usage can be found in NSQL Queries used in the PMO Accelerator portlets
For example, the NSQL Query ID = cop.currentIssues utilizes both the @NVL@ (to check for null value) and @SYSDATE@ (the database system date). Query ID = cop.prjCostEffortDashboard has an example of the concatenation (@+@) symbol.
Hope this Helps!