Plug-in, add-in, add-on, extension OR snap-in
All these are similar in nature, they are regarded as the kind of module that interacts with another software) to add specific function [add extra Features] or to support specific file format/device and to add features to a software with limited features.
The terminologies that could be used in place of the above are not limited to these, it may be called any other name in any other software. For example, in Photoshop, we have terms such as brush, style, action...
In a simple term, I Call them ADDITIVES.
This scenario will be explained further in course scope 3 (THE DOCUMENT)
Hybridised
When packages are in combined state of the previously mentioned (of different OS, as a PC software, as a mobile apps, even as in emulators, existing both as offline/local package OR as a cloud/online package.
For example, Microsoft Office packages have it from in offline package as well as online features within it; it cloud feature could be used as standalone without even the application/software, all you need is to log-in to Me office website through any browser using our account details. It also transcend through PC software or rather make a choice of using a mobile apps (smartphones/tablet).
Many other software is also constructed in this manner eg Adobe, Google apps.
File format (type/extension)
File in computing goes beyond pieces of papers but almost everything we can even observe as long as software is concerned. Even software program) is a kind of file. In Android we have the apk, window we have exe/msi,... In Java, we have jar, jad.
File is of different type which may be based on area where it maybe applicable, we have images, audio, books, videos, 3D, application software, GIS, file archivers/compressors, textual and document, scripts and code... etc. File type deals with such file performs.
File extension is the sub-type of the file type which performs similar function. It is identified by "dot" extension eg. .jpg, .png, .gif
All the above examples are of the same type i.e. they are all image file type, but each have it unique properties that is peculiar to it. Some software support some format/extension, while some don't (in terms of reading and writing)
Other extension include .pdf, .doc, .docx, .ppt, .epub, .mp3, .mp4, .mkv, .aac, .psd, .cdr, .iso, .zip, .rar, .ico... etc.
NOTE: It should be noted that this extension is different from the extension in terms of Plug-in and Co.