No, the articles aren't redundant.

You could say Write T or Write F without articles, because those would just be letters which you write on the paper.

However, the person who wrote these instructions used the verb place, which suggests that 'T' and 'F' are 'things' that can be moved around to different positions. That's why they have articles.

NB I actually think the choice of the verb 'place' is a little odd, especially as you're not told where to place these letters. I'd have used 'write'.

Thanks Liseuse

Aha, write T and place a T! That makes sense :)

The pronunciation of "f" is "ef", so it starts with a vowel and the article "a" gets an "n" added.

Thanks Mr. Bernhard

But sorry, the way I asked seemed not good.

My question is; is it necessary to put 'a' before T? In other words, should this T be a countable word?