QUESTION
Along with the QCC file, there are other files with the same names but different extensions such as QCN, EIF, ET3, NTS, QCG and RTF.
ANSWER
The short answer is that the QCC file cannot be changed within the same version in order to be compatible with earlier versions of QC-CALC. As such, we had to create extra file extensions to store information about new features as they are developed. Most of the time, extra information in the external files is combined into the QCC file when the next version is released.
NOTE: These files should be left in a directory and not deleted. QC-CALC automatically creates and deletes the files as it needs them. Deleting them could cause a loss of data or functionality.
Here are the specific file extensions and their usage:
QCN - Many years ago, this started out as a place to store some notes, but now it is literally used as overflow. Dim Source & Extra Info are in there as is overflow for long calculations in calculated dimension. File specifc (vs. global) reporting/exporting settings are also stored in this file. With each major version, we tend to move these settings into the QCC file and then it builds up again as the version expands to new features.
EIF - This is a flag file from EDL telling QC-CALC to log changes to the data. QC-CALC has this ability built in, but does not log changes by default. The EIF file is effectively a switch that turns the behavior on. If you are using EDL to load the data to SQL Server and want to load a particular QCC file (example: widget.qcc), EDL will create a file called widget.eif. Whenever a change occurs to the data, QC-CALC looks to see if this file exists. If it does, then it logs the change to the ET3 file of the same name. If it does not, no changes are logged.
ET3 - This is the machine readable transaction log of changes that occur to the corresponding QCC file. As changes occur to a QCC file, if a corresponding EIF file exists for that QCC file, an ET3 file is created and changes are logged into this file until the next time EDL is scheduled to import the QCC file. This allows EDL to keep track of changes that occur without having to compare every record in the QCC file to the corresponding one in the SQL database. The ET3 file is deleted once EDL has read it so it isn't repeatedly picking up the same changes.
QCG - This is the Group file from QC-CALC SPC. When you create a "Group" in QC-CALC SPC this file is created. It stores the names of the individual .qcc file within the group and some settings for those files and the group as a whole (like filters depending on the actual settings for filters in QC-CALC SPC). - This file is not associated with any QCC file.
NTS - This file stores point and record level notes that uses enter in QC-CALC.
P11 - All 21 CFR Part 11 changes (audit trail) are stored in this external file.
RTF - This is the RTF file that is used for the Notes Input for reports. It is rarely used.
PMF - This will only exist if the Manual Entry screen is being used in QC-CALC SPC. This is not to be confused with the Manual Input screen that exists in QC-CALC Real-Time. It contains the mappings for all of the groups and sets in the Manual Entry screen.
ETF - This is an older version of the ET3 file. As of 2011, it was deprecated and is no longer used.