Mishna Berura Source Sheet 161:1

It is important to note that the presence of a chatzitzah is different than one who didn't immerse a part of his body at all. In the latter case, the tevilla is totally lacking since a part of the body is not immersed.

On the other hand, by chatztitah, even if the chatzitzah is totally blocking a part of the body from water entering there, it is only an issue (MidOraysa) if it the following two conditions are met:

1. It is on most of the body

2. One is makpid on its presence.

The reason is because when the chatzitzah is negligible, the chatztizah is batel to the body and it is as though that immersed body part was toiveled.

After his introduction of the succession of the Rabbinic enactments onto the DeOraysa in the realm of Tevilas Mikva, Mishna Berura explains how this translates into Netilas Yadayim, yin sham.

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This is referring to the dirt that can accumulate under ones fingernails. It is not a chatzitah since it is both only on miut haguf and one is not makpid on its presence.

Mishna Berura says this cannot be referring to actual "tzoah" for two distinct reasons.

1. A reason related to Hilchos Netilas Yadayim

2. A reason related to Hilchos Chatztiza.

1. A reason related to Hilchos Netilas Yadayim

The entire purpose of netilas yadayim, we said in the beginning of Hil. Netilas Yadayim, was because of yadayim askaniyos.

Since this tzoah is constantly on his hands, the entire point of the Netila to clean away that dirtiness is not being accomplished.

2. A reason related to Hilchos Chatzitzah.

The other reason why the Shulchan Aruch is probably not referring to actual tzoah is because one will certainly be makpid of the presence of putrid tzoah on his hands, thus making a chatzitzah.

Introduction to next part of the Shulchan Aruch:

The Shulchan Aruch goes into the actual substances of dirt which may or may not be a chatzitzah. There are chiefly 2 categories of dirt:

1. Fingernail Sweat/Lightly Adhesive Dirt: Dirt that accumulates "internally" from the sweat of fingernails. (This halacha is special to the area around the fingernails, and will make a difference where on the fingernail it is found)

2. Potter's Clay/Strongly Adhesive Dirt: If one comes in contact with potter's clay, an accumulation can build up.(This halacha applies to any part of the hand where this clay is found, no differences are drawn, unless it is very minute)

The reading of this Seif is very interesting, especially regarding the halachos of dirt. First we mention tzoah not under the skin, which we say is a chatztitah.

Although in Orach Chaim by Hil. Netilas Yadayim the Shulchan Aruch doesn't spell out the differences between kneged habasar and not kneged habasar, in Yoreh Deah, by Hil. Tevila, the Shulchan Aruch does:

As we mentioned in the hakdama and throughout the seif, it is important to identify which type of "dirt" we are discussing when we are discussing dirt. That said, the Rema, coming off of the Mechaber's lenient psak says a very cryptic Lashon:

הגה ומשום הכי לא נהגו לנקר הטיט שתחת הצפרנים

It isn't clear what type of dirt he is discussing (just says הטיט) nor does he state in clear terms where it is found! (תחת הצפרנים, doesn't say if kneged habasar or not)

If so, in what case exactly is the Rema stating the leniency?

In explanation of the Rema the Magen Avraham writes:

The Magen Avraham seems to be saying that the Rema is being maykel by all cases of dirt (even adhesive dirt) as long as it is kneged habasar, since most people aren't makpid on such a small amount.

But shelo kneged habasar, it will be a chatzitzah even by weakly adhesive dirt, since most people will be makpid on that.

However some argue to be maykel even if it is on the fingernail shelo kneged habasar if it is a weakly adhesive dirt / fingernail sweat.

The reading of the Taz is geshmak. He notes an interesting thing:

The Mechaber in his work "Beis Yosef" does bring the modification of Rabbeinu Tam that only if it is adhesive dirt is it a problem if it is shelo kneghed habasar. But in Beis Yosef's work "Shulchan Aruch" he doesn't bring this kula!

Then he goes ahead to say that although there is no apparent answer as to why in Shulchan Aruch this isn't mentioned, it's still clear from Hil. Netilas Yadayim that this kula exists.