These 2 seifim respresent part of the laws relating to Teliya - attribution of stains to something other than menstrual flow.  As we will see, the Rabbis have established significant leniencies in this area and have constructed the halacha in such a way that the stains can be disregarded if there is virtually any possibility that it came from an outside source.  The specific examples used in the Shulchan Aruch are demonstrative only; they represent possibilities of staining that could arise based on the normal clothing and on the normal daily activities of a woman of that time. The Shulchan Aruch does not, for example, give consideration to the type of underwear that women typically wear today.  The example that the Taz (10) uses to explain the law regarding a stain on a woman's foot is of a woman "sitting in the manner of the Ishmaelites, with her legs crossed underneath herself."  Thus, the exemption for the outside of the foot or thigh makes a certain amount of sense.  Additionally, since women did not wear undergarments as we have, there was no physical barrier between her genitals and her lower body.  Again, this supports the notion that a stain on the inner part of her leg would be impure, while one on the outer part would be pure.

The halacha also recognizes physical reality - that a woman cannot reasonably expect a stain that is on her shoulder to from uterine blood.  Thus, there is a general exemption for stains, even those found on her torso.  There is a general exception to that exemption that if the woman had behaved in such a way that her feet were over her head, or her torso in contact with her legs such as in yoga or gymnastics, the stain would still be considered impure.

We do not consider the possibility that the woman might have had impure blood on her hands and then transferred that to yet another part of the body because of a general halachic principle called "lo mechazkinan tumah me makom le mekom - we do not make assumptions about the movement of impurities from one place to the other.  However, the stain on her hands would render her impure (in the absence of another attribution) because of the stronger principle of "yadaim askanios hen" - peoples' hands regularly touch all parts of their bodies and it is possible that her hand was in contact with her vaginal area.  Theoretically, if a woman could not move her hands for any reason and could therefore not possibly have touched herself anywhere, a stain on her hand would not render her impure.

The halacha also recognizes that there is a basic difference between a stain that is only on a woman's body and one that is on her clothing as well, since blood that is only on her skin is much more probable to be from the woman's body itself than from an outside source.  Therefore, it is more willing to accept an attribution to an outside source if the stain is on her clothing as well.  The only time a stain solely on the skin can be attributed to other causes is if she has a wound that could possibly have bled into the area of the stain.

In the case of the stain being on her clothes as well, we expand the possible attributions to include the woman's dealing with blood during her normal activities regardless of whether the stain is above the belt or below the beltline.

Seif 12 continues on the topic of attribution.  It starts off by telling us that a stain on clothing when there is no attribution possible is impure.  It further tells us that that the halacha is even more lenient in a case where the stain is only on her clothing.  In that case, there is a distinction between whether the stain is below the belt or above the belt.  If there is no source for attribution, then a stain below the belt is impure, while a stain above the belt would be pure. 

In summary, if the stain appears on the skin only (unless there is a wound) she is considered impure regardless of its location.  If the stain appears on both her skin and her clothing, if there is no external source of attribution she is impure, whether the stain is above or below the belt and is pure if there is an external attribution.  A stain only on a woman's clothing is always pure, unless it is both below the belt line and has no possible external attribution.

Clearly, the various rules of attribution need to be considered in light of both normal daily activity and normal clothing today.  If a woman finds a stain on her thigh but none on her underwear (assuming she didn't remove it at any time), the stain clearly can not be from uterine bleeding.  Similarly, although the halacha recognizes that a stain from an outside source might get into an inner layer of clothing, that stain is only attributed to an outside source if it is physically possible for the stain to have gotten there based on the clothing she is wearing.