Stella model files use the XMILE standard to describe the diagrams, equations and interface. Files with extension .stmx or .itmx (a convention for iThink) are saved in this format. There is no difference between .stmx and .itmx files except for the extension.
When working with .stmx or .itmx files there may also be a .isdb file with the same name. This file contains all the runs associated with the model. The file is saved automatically when you save a model. If there are not runs, as will happen after restoring outputs, there is no .isdb file. The .isdb file is an SQLite database with results stored in blobs. It can be queried using tools supporting SQLite, but there is usually no need to look inside a .isdb file. Stella Enterprise also supports a binary format data file with extension .isdbb which is faster and more compact than the .isdb file and is intended for very large dataset.
Stella can use a variety of additional files for import and export as well as supporting graphics and media files. These files are all standard formats and can be created by a number of different application. When Stella creates a .csv file it does so using the delimiter based on your selection from the Preferences Dialog. When Stella reads a .csv file it determines the delimiters in use based on the file content.
If the models have modules that are saved as separate files, these modules will be put in a folder that has the same name as the model followed by _Modules. For example, the model Workforce01.stmx would have modules save in the subfolder Workforce01_Modules. Each of the modules is a .itmx file (a convention), that can be opened independently of the parent model.
Stella has a bundled format file with extension .stmz. With the bundled format all supporting files, include import and export files, are together in the single .stmz file.
Stella's .stmz files are simply zipped file archives. They contain a .stmx model file and potentially a data file in the main folder, and then supporting files in the subfolders Data, Graphics, Sounds, and Movies. If the model has modules saved in separate files there may also be a modelname_Modules folder as described above.
There is also a file named .stmx_model in a bundled format archive that simply contains the name of the model file. This is a simple convention to validate that the archive is intended to be a .stmz file.
You can rename a .stmz file to have extension .zip and open it using any archiving tool, including those built into the operating system of the computer you are working with. Once expanded, you can open the contained model just as you could open any other model.
When you open a .stmz file you may be asked to specify a Data Folder. This folder will contain files that are in the Data subfolder of the archive. Stella will use files in this folder rather than files in the archive for importing and exporting data. When a file does not exist in the specified Data Folder a copy of the file in the archive will be placed there. When a file already exists, it will be used instead of the file in the archive. To update a file in the Data Folder to the content in the archive simply delete it before opening the .stmz file.
Even though .stmz files are zipped archives, there is no guarantee that saving a .stmz file will contain all the supporting files that the original .stmz file contained. When .stmz files are saved, only the supporting material required for the model will be included. If you have stopped using a graphics or other file it will no longer be included in the archive. To preserve all files manually archiving the file structure you have in place will be the most effective approach. To have this work well, ensure that all referenced files are contained in subfolders of the model folder.
When saving to a bundled format from the File Menu you can also save directly to a .zip archive. The content of that .zip archive will be identical to the content of the .stmz archive, except there will not be a filed named .stmz containing the model name. To use a .zip archive in Stella first expand it, then open the .stmx file directly.