There are some parts of TI-BASIC files that I don't understand. By not understanding, I mean I don't know what they represent or do.

In the first image, I outlined the parts I don't understand with white rectangles. If anyone could, I would appreciate it if they explained those.
If anyone has a document or web-page that explains the formatting of .8xp files that also would be helpful.

http://merthsoft.com/linkguide/ti83+/fformat.html

This guide should explain everything you need

Wow. That link really explains a lot about ti-files, and it gives viewers everything they want to know about calculator files. That can be useful when a calculator user is trying to figure out what files to download, or what different files do when they download them.

That looks awesome! I should look at that sometime.

One question: Is there a guide like that for the 85?

*RogerWilco runs

If you want to look at an implementation of data->string / string->data, there's one in C++ here:
https://github.com/adriweb/tivars_lib_cpp/blob/master/src/TypeHandlers/TH_Tokenized.cpp#L107-L130

For the other var types and subtypes, look at the files here: https://github.com/adriweb/tivars_lib_cpp/tree/master/src/TypeHandlers

Yea, http://merthsoft.com/linkguide/ti85/index.html

Thanks!

Disclaimer: The answer is (probably) extremely obvious.

I've been writing a prototype for a TI-BASIC editor on PC before I continue to create one for on-calc use, but I've run into some problems.
Merthsoft states that the checksum is "...the lower 16 bits of the sum of all bytes in the data section."
I tried to loop through each byte and add that to the total, but I didn't get the same number. I probably misread that, but can't find what I did wrong. Currently, the only problem is the checksum. Once I get that sorted out I expect I'll be able to finish up the prototype and start making another editor.
How is the checksum calculated?

I apologize for such a dumb question.

I'm sure you know this, but the on-calc format is simpler so you won't have to compute checksums on calc.
Anyway some pseudocode to compute the checksum:
Code:

As I expected, there was a very simple answer.
After some tinkering, I got it to output this result:
"0241"
and when compared with the working file's checksum, 4102,t he answer becomes painfully obvious. Thank you for responding jacobly, also I was not aware that it's easier to create programs on-calc. Well, at least it works now.

I did this in python 3 once.
Here's the pastebin

This was based off of This amazing program, special thanks to keyoni29!