What is the difference between a text indicating a prohibitory (tahrim) command and a text indicating a discretionary (tanzih) command? And what is the legal ruling for that which is prohibited by way of tanzih?

The basic principle concerning a prohibition is that it signifies حرمة (prohibition) unless there is an indication that diverts it from this principle and shows that it is for تنزيه (discommendation).

Al-Shafi'i said: "Whatever Allah has prohibited is unlawful until there is an indication that the prohibition is not for unlawfulness, and that what is intended by it is guidance, discommendation, or etiquette for what is prohibited. And whatever the Messenger of Allah, peace and blessings be upon him, has prohibited is likewise."

He also said: "The fundamental principle concerning the prohibition from the Messenger of Allah, peace and blessings be upon him, is that everything he prohibited is unlawful until an indication comes from him showing that he only prohibited it for a meaning other than unlawfulness, either he intended to prohibit some matters but not others, or he intended the prohibition for discommendation of what is prohibited, and for etiquette and choice."

Shaykh Ibn Baz clarified that the basic principle concerning a prohibition is unlawfulness, and it is not shifted from unlawfulness to disfavor (كراهة) except with evidence. So, if something is prohibited and then performed, it indicates that the prohibition is for disfavor, such as the Prophet's prohibition, peace and blessings be upon him, against drinking while standing, and then him drinking while standing on some occasions.