All bloodstains that come from Rekem are clean.
Rabbi Judah declares them unclean, because the people who live there are proselytes though misguided.
Those that come from non-Jews are clean.
Those that come from Israelites or from Samaritans: Rabbi Meir declares them unclean, But the sages declare them clean because they are not suspected in regard to their stains.
Today's mishnah deals with clothing that is found with a bloodstain on it. The question is do we have to be concerned that the blood is impure and the clothing defiles?
Section one: Rekem is on the eastern border of the land of Israel (see Gittin 1:1-2). Its residents, according to the first opinion, are considered to be non-Jews. Since non-Jews don't defile, their bloodstained clothing is also not considered to be defiling.
Rabbi Judah says that the people of Rekem are actually converts.
It is interesting that in this mishnah we have an argument over the status of these people the first opinion holds that their conversion was not valid, whereas Rabbi Judah holds that their conversion was valid. We can sense, perhaps, that the laws of conversion were still in flux.
Despite their conversion, Rabbi Judah holds that they do not properly observe the laws of menstrual purity and therefore their stains are impure. As we shall see below, the sages hold that bloodstained cloaks of Israelites do not defile. These people converted but didn't learn the laws all that well. That is likely why the other rabbis consider them not to be Jewish.
Section two: As stated above, non-Israelites do not defile, at least not through their menstrual blood. Therefore, it is obvious that their bloodstained clothing is not impure.
Section three: Israelites and Samaritans are both subject to the laws of menstrual purity meaning their menstrual blood is impure. According to Rabbi Meir, if one finds a cloak from an Israelite or Samaritan with a bloodstain on it, one must be concerned that it is menstrual blood. Therefore, it defiles.
The other sages say that Israelites and Samaritans are not suspected of simply giving clothing stained with menstrual blood to others. In other words, they are cautious about the laws of menstrual purity. Therefore, if we find blood on their clothing we can assume that it is not menstrual blood. The clothing is pure.