18. Since the laws of staining is Rabbinic, we are lenient in them and attribute a stain to anything that we possibly can.  For example, if she slaughtered a domesticated or wild animal or a bird or if she had been working with blood stains, or was sitting next to people who were doing so, or she passed through the meat market and we find a stain on her garments, she is pure.  This is true even if she was wearing 3 layers of clothing and the stain was found on the innermost one.  However, if the stain is found on her body we do not make any attribution unless she has a wound so long as it is in a place that can reasonably explain the blood stain.  Even if she is healed, if she could possibly scratch it and cause it to reopen, we will use the wound for attribution, even if right now she has a scab and there is dripping from the wound.
     

19. As we find areas of attribution for her, we will also use for attribution blood that is on her son or husband, if they were dealing with blood stains or have wounds, because it is usual for them to be in contact with her.  However, if they were working with blood and there was no blood on them, then we cannot make the attribution, unless they had been working with something that sprays blood, such as slaughtering and the like.  And if she slept in a bed with several women who had wounds, we can use that for attribution as with her son and husband
     

20. If someone normally releases blood from his penis and during intercourse she finds a stain on her cloth, the stain is attributed to her husband.
     

22. If she does not know if she passed through the meat market or whether she was with people dealing with stains she cannot attribute her stains to those things.  This restriction applies only if the butchers or other sources of stain have a known district or location.  But if they are spread out randomly we do attribute to those causes because it's possible that she walked by one of the areas and didn't notice.
     

23. If she dealt with something red and found a black stain or the other way around we do not permit attribution.  This applies only when the combination is black/red or red/black.  However, if the difference is in shades of black or shades of red even if the colors aren't a perfect match.  If she was working with vegetable juice, meat juices or red ointments she can use those for attribution.
     

31. If she finds a stain and does not have anything to attribute it to and we are uncertain if the stain is blood or not, we apply tests with 7 substances.  If these do not dissolve the stain, then it is not blood and she is pure.  If she does not apply the substances she is impure out of doubt. Today we do not have the 7 substance test since we are uncertain as to what they all are.

 

What's Going On - More on Attribution

As we have previously learned and as is directly stated in Seif 18, we endeavor to be as lenient as reasonably possible in the area of stains.  As an aside, an interesting point to note is that the Shulchan Aruch here is stating specifically that women are permitted to slaughter animals - this is an issue that is debated among the Rishonim at some length.