Storing a palette location and size...

You can store data in an app label -- as Brian suggests -- but I prefer to write it to a file because I use scripts in three or more versions of InDesign. Using app. labels wouldn't work very well then. Another advantage of using a file is that the content is easier to inspect and easier to reset/delete/change manually.

I use this genelal schema for working with config files. The config file is names after the script (so abc.jsx creates a config file abc.txt) and is stored in the script's folder. You can use a separate Reseources folder, that's probably neater.

The below script stores the window's location, but also the content of the two text fields. You can store anything you like: location, size (in case of resizeable windows), selected items in lists, selected radiobuttons and checkboxes, etc. etc. And restore the window's slections.

The file that's created looks like this:

({location:[317, 385], first:"Piet", last:"Vlerk"})

Here's the script:

function scriptPath () {
  try {
    return app.activeScript;
  } catch (e) {
    return File (e.fileName);
  }
}

// Write the config file
function saveData (obj) {
  var f = File (scriptPath().fullName.replace (/\.jsx?(bin)?$/, '.txt'));
  f.open ('w');
  f.write (obj.toSource());
  f.close ();
}

// Read the config file
function getPrevious () {
  var f = File (scriptPath().fullName.replace (/\.jsx?(bin)?$/, '.txt'));
  try {
    return $.evalFile(f);
  } catch (_) {
    return null;
  }
}

var w = new Window ('dialog');
  w.firstName = w.add ('edittext {characters: 10}');
  w.lastName = w.add ('edittext {characters: 10}');
  w.add ('button {text: "OK"}');
  w.add ('button {text: "Cancel"}');
  
w.onShow = function () {
  // Look for the config file
  var previous = getPrevious();
  // It was there, populate window
  if (previous !== null) {
    w.location = previous.location,
    w.firstName.text = previous.first,
    w.lastName.text = previous.last
  }
  w.firstName.active = true;
}

// OK clicked: store the window data
if (w.show() === 1) {
  saveData ({
    location: [w.location.x, w.location.y],
    first: w.firstName.text,
    last: w.lastName.text
  });
}