Short answer:File format.
Long answer:
We need to first understand what exactly is a file format.Every file stores data (in binaries, 0s and 1s). The data needs to be structured in a certain way such that Applications can open them.
Follow the ASCII table and you get a ASCII encoded text file (.txt), where applications like notepad can open them and display them as letters in your screen. Follow the html file format and you get a HTML file (.html), where browser applications like google chrome can open them as webpages.
XML is also a file format - the purpose of which is to store data. If you open a .xml file, you can see every objects you created in the ALE in .xml. XML is a standard file format, used by many other applications around the world. Some programs communicate with each other using the .xml file format.
XML is
readable. That means, the data is encoded into letters such that we humans can read the data too. This in turn sacrifices optimization = lesser amount of data can be used to achieve the same results.
Therefore, I assume Eric Gurt came up with his own file format, a .pb2map file format. It is
not readable, as the data are not encoded in a way other applications can represent the data with letters. Standard applications don't understand how to open this file - with the exception of his web application. His web application will read the .pb2map file, and perform corresponding actions to generate the map according to the data in it. (He programmed the web application to perform these taskes.)
We are given two options to download a PB2 map file, a .xml file or a .pb2map file. The .xml file is for us to take a look at how the data is structured and the .pb2map file is the more optimized version representing PB2 maps. I assume the web application will convert the .xml file into .pb2file if you upload the .xml file.
More reading:
File format:
https://en.wikipedia.org/wiki/File_formatXML:
https://en.wikipedia.org/wiki/XML#:~:te ... age%20(XML)
P.S: There is a forum topic which attempts to decipher the meaning of .pb2map file.