Use of the Subjunctive in This Sentence

[Bobbert]

I saw this sentence written on a reputable web site, so I'm assuming it is written correctly:

Una de las reglas es que escribas tu nombre completo.

I can't figure out why the subjunctive form of "escribir" was used. Would it be incorrect to use the indicative form?

Grammar examples and/or explanations are appreciated.

[poli]

When que is followed by a conjugated verb the verb will be subjunctive if it implies a wish. Another example would be que viva Mexico or que esté bien. It's like ojalá que...

[Bobbert]

Thanks, poli. I would never have thought of this as a "wish," so I guess I'll have to expand my thinking a little and think about what the sentence is implying and not just what the words in the sentence are.

[AngelicaDeAlquezar]

The subjunctive here is needed because there is some expectation that you will follow the rule, something you haven't done yet. It's the notion of something happening in the future that needs the subjunctive. As Poli said, you can take the hint from the "que", but also the context will tell what mode you need.
You cannot use the indicative because then the sentence would be stating a fact. It's been observed that you always write your full name, so if I say: "La regla es que escribes tu nombre completo", it means you do this all the time, and it has become a rule for you. If I use the subjunctive, I'm asking you to follow the general rule and write your full name.

[Bobbert]

Thank you, AngelicadeAlquezar and poli. That clears it up for me.