بسم الله الرحمن الرحيم
In the name of God the Merciful
It is to be taken from everyone getting money much more than his annual requirement
: In Iraq the year 1965 it was estimated by the late interpreter
Mohammed-Ali Hassan al-Hilly

(died at 1991 AD)
that it is every gain of 50 Iraqi dinars or more (at that time the dinar equalled 3 dollars, and the dollar at that time was a powerful currency not like it is today.) This means any gain more than about 150 dollars or more at the year 1965.

It is taken from the owners of factories, companies, mines, large supermarkets ...etc if they gain much more than their requirements.

If one finds a treasure --> he has to pay the fifth or 20% of his treasure.
If one finds a mine and exploits it and it gives him much money gain --> he has to pay the fifth or 20% of his gain.
If a man had a company or trade and all of a sudden he gets much increase gain out of his work --> he has to pay the fifth or 20% of his gain.
=============================================

The distribution of the "fifth" tax

It is mentioned in the Quran 8: 41

وَاعْلَمُواْ أَنَّمَا غَنِمْتُم مِّن شَيْءٍفَأَنَّ لِلّهِ خُمُسَهُ وَلِلرَّسُولِ وَلِذِي الْقُرْبَى وَالْيَتَامَىوَالْمَسَاكِ ينِ وَابْنِ السَّبِيلِ إِن كُنتُمْ آمَنتُمْ بِاللّهِ وَمَاأَنزَلْنَا عَلَى عَبْدِنَا يَوْمَ الْفُرْقَانِ يَوْمَ الْتَقَى الْجَمْعَانِوَاللّهُ عَلَى كُلِّ شَيْءٍ قَدِيرٌ

Theexplanation:

(And know that anything you [Muslims] seize as a spoil; a fifth of it is due to [6 shares]:

God [this share is spent for the mosques],
and the apostle [this share is given to the oraters in mosques],
and the kindred [this share is given to the poor among your relatives]
and orphans,
and the needy
and the wayfarer;
if you believe in God [and His reward and punishment] and [in the revelation and victory] that We have revealed to [Mohammed:] Our servant on the day of the 'Furqan' [: or the revelation of frequent portions of Quranic ayat]; the day when the two parties [: the Muslims and the idolaters] met [to fight]; and God is All-Able to do anything [: so that He will make you victorious if you obey God and the messenger, and will forsake you if you disobey.] )

--

The interpreter said: If you sell something by 200 dinars,then you sell it few days later by 250 dinars then you have gained 50 dinars,so you have to pay 10 dinars as the fifth tax; and as such every gain which youdo not make any effort in gaining it you have to pay its fifth. This is after you take out of it, your requirement for 3 months, then from the rest you take one fifth and expend it for the sake of God in six shares as explaine