(יט) וְאִשָּׁה כִּי תִהְיֶה זָבָה דָּם יִהְיֶה זֹבָהּ בִּבְשָׂרָהּ שִׁבְעַת יָמִים תִּהְיֶה בְנִדָּתָהּ וְכָל הַנֹּגֵעַ בָּהּ יִטְמָא עַד הָעָרֶב.
(19) And if a woman has a blood flow, flowing from her flesh, she should be in her niddah state for seven days; and whoever touches her will be ritually impure until the evening.
(כה) וְאִשָּׁ֡ה כִּֽי־יָזוּב֩ ז֨וֹב דָּמָ֜הּ יָמִ֣ים רַבִּ֗ים בְּלֹא֙ עֶת־נִדָּתָ֔הּ א֥וֹ כִֽי־תָז֖וּב עַל־נִדָּתָ֑הּ כָּל־יְמֵ֞י ז֣וֹב טֻמְאָתָ֗הּ כִּימֵ֧י נִדָּתָ֛הּ תִּהְיֶ֖ה טְמֵאָ֥ה הִֽוא׃ (כו) כָּל־הַמִּשְׁכָּ֞ב אֲשֶׁר־תִּשְׁכַּ֤ב עָלָיו֙ כָּל־יְמֵ֣י זוֹבָ֔הּ כְּמִשְׁכַּ֥ב נִדָּתָ֖הּ יִֽהְיֶה־לָּ֑הּ וְכָֽל־הַכְּלִי֙ אֲשֶׁ֣ר תֵּשֵׁ֣ב עָלָ֔יו טָמֵ֣א יִהְיֶ֔ה כְּטֻמְאַ֖ת נִדָּתָֽהּ׃ (כז) וְכָל־הַנּוֹגֵ֥עַ בָּ֖ם יִטְמָ֑א וְכִבֶּ֧ס בְּגָדָ֛יו וְרָחַ֥ץ בַּמַּ֖יִם וְטָמֵ֥א עַד־הָעָֽרֶב׃ (כח) וְאִֽם־טָהֲרָ֖ה מִזּוֹבָ֑הּ וְסָ֥פְרָה לָּ֛הּ שִׁבְעַ֥ת יָמִ֖ים וְאַחַ֥ר תִּטְהָֽר׃
(25) And if a woman have an issue of her blood many days not in the time of her impurity, or if she have an issue beyond the time of her impurity; all the days of the issue of her uncleanness she shall be as in the days of her impurity: she is unclean. (26) Every bed whereon she lieth all the days of her issue shall be unto her as the bed of her impurity; and every thing whereon she sitteth shall be unclean, as the uncleanness of her impurity. (27) And whosoever toucheth those things shall be unclean, and shall wash his clothes, and bathe himself in water, and be unclean until the even. (28) But if she be cleansed of her issue, then she shall number to herself seven days, and after that she shall be clean.
Why do you think there are 2 different categories?
Why do you think sex is prohibited in each case?
אמר ר' זירא בנות ישראל החמירו על עצמן שאפילו רואות טפת דם כחרדל יושבות עליה שבעה נקיים
...R' Zeira said: The daughters of Israel took a stringency upon themselves that if they saw even a drop of blood the size of a mustard seed, they would wait 7 clean days.
Why do you think this practice evolved in this way?
משרבו הגלוית ותכפו הצרות ונמעתו הלבבות חשו שמא יבאו לטעות באיסור כרת שמא רתאה אשה בימי נדתה ששה
ימים ויהיה הכל דם טהור ובשביעי שמא תראה דם טמא וסברה לטבול בליל שמיני וצריכה עוד שבעה ימים החמירו
לטמא כל מראה דם אדום וכדי שלא יבואו לידי טעות בין ימי נדה וימי זיבה הוסיפו חומרא אחר חומרא עד שאמרו
שאפילו אם לא תראה אלא טיפת דם כחרדל תשב עליה ז‘ נקיים כזבה גדולה
Tur Yoreh Deah 183 (Compilation of halachic/legal practice by Yaakov Ben Asher, 13th century)
Since we've had many exiles, and troubles multiplied, and our hearts have shrunken, they suspected that people would make mistakes and come to this transgression, which results in the punishment of "karet" (spiritual cutting-off), lest a woman would see "tahor"(pure) blood for 6 days during her "niddah" days, and on the 7th day maybe she would see "tameh"(impure) blood and she would believe that she should immerse on the 8th night. However, she would need 7 more days. They were stringent and decided all blood that appeared red was "tameh"(impure). And so they would not make a mistake between "niddah" days and "zavah" days, they added a stringency upon the stringency until they said that even if you only see a drop of blood the size of a mustard seed, she should sit 7 clean days like a "zavah gedola" (major zavah).
Conservative authorities teach that the laws of family purity are normative and still in force, including the requirement to refrain from sexual relations during niddah, yet there is a difference of opinions over how much other strictures need to be observed, such as whether there should be complete prohibition on any touching during niddah and whether women are required to count seven "clean" days before immersing in the mikvah.
In December 2006, the Rabbinical Assembly's Committee on Jewish Law and Standards passed three responsa discussing the extent of Biblical requirements and continuing applicability rabbinic prohibitions concerning niddah for Conservative Jews. Each responsum advocated different standards of observance; two responsa were passed as majority opinions, the third responsum was passed as a minority opinion.
According to the two majority opinions, the "seven clean days" need not be observed today and women may immerse and resume sexual relations after seven days from the beginning of menstruation, or after its cessation, if it lasts longer than seven days. Rabbi Grossman, a majority opinion, and Rabbi Berkowitz, the minority opinion, ruled that women may rely on their own discretion about when menstruation has ended, and need not routinely engage in bedikah as described above.