I'm not 100% sure I get the question but I'll take a stab at it:
It's in the JS file for the plugin. Plugin Parameters are special entries in the JavaScript comments section near the top. They look like this:
* @param Screen Height
* @desc Adjusts the height of the screen.
* Default: 624
* @default 624
That's just a comment in JavaScript but in RMMV it shows up as a parameter in the Plugin Manager called "Screen Height". The @param part tells RMMV to make a new parameter. The @desc part tells RMMV to put the following text as the description. The third line has no @ symbol and does nothing in RMMV, it's just a note Yanfly added, probably just for developers so they know what the normal default is in case they change it. The fourth line @default tells RMMV what the default value for that parameter should be.
To access this parameter from JavaScript, you have to use the built-in Plugin Manager function that's a part of the core scripts. Most plugin writers make new javascript variables and store these values in them for use later. Here's how Yanfly does it in the Core Engine:
Yanfly.Parameters = PluginManager.parameters('YEP_CoreEngine');
Yanfly.Param = Yanfly.Param || {};
Yanfly.Icon = Yanfly.Icon || {}; // Ignore this line, it's not part of our lesson.
Yanfly.Param.ScaleBattleback = String(Yanfly.Parameters['Scale Battlebacks']);
That first line is the most important one. Yanfly uses RMMV's PluginManager.parameters() function and then inserts the name of the plugin. It will return a list of all the parameters that were defined in the comments section of that specific plugin. Yanfly then sets the value of a variable called Parameters to the result. The Parameters object is a property inside of the Yanfly object.
Finally, for convenience, Yanfly creates a new object called Param and then proceeds to add a new property to it for each parameter, getting the parameter value with Yanfly.Parameters['parameter-name']. It may seem strange but the reason for the bracket notation instead of dot notation is because parameter names can have any sort of special character like a space or a dash. JavaScript property names are allowed to have just about any character you want but only phrases starting with an underscore or regular letters and only containing letters and numbers can be accessed via dot notation. If the property name has special characters, you have to put the name in quotation marks and access it with bracket notation.
So these two are the exact same:
window.document; // The document Object which is a property of the window object.
window["document"]; // The document Object which is a property of the window object.
window.document.body; // The HTML <BODY> Tag
window["document"]["body"]; // The HTML<BODY> Tag.
window.document["body"]; // The HTML <BODY> Tag
// This demonstrates the 2 major ways of accessing object's properties in JavaScript.
The TL;DR Answer:
Use the PluginManager.parameters('Your-Plugin-Name'); function, which is a part of the core RMMV scripts. That will return an object that contains all of your plugin parameters by name. Access them via bracket notation if they have any special characters like spaces or dashes.
