In general, there are two fundamental approaches to the laws of kashrut on Pesaḥ. According to most poskim, the laws of ḥametz on Pesaḥ are no different than the laws of all other forbidden foods, with one exception: all other forbidden foods are batel be-shishim (rendered insignificant when constituting less than one sixtieth of the volume of a mixture), whereas ḥametz is not. However, all other laws of mixtures apply to ḥametz on Pesaḥ. Therefore, when there is no halakhic reason to suspect that a food mixture has absorbed the taste of ḥametz, it is kosher for Pesaḥ. Likewise, where an individual posek is stringent and the great majority of poskim are lenient, halakha follows the lenient opinion.
However, Ashkenazim are customarily very strict about ḥametz, often showing concern for a stringent opinion even against the lenient majority and practicing caution where general halakhic principles indicate no reason to do so. Nevertheless, Ashkenazic custom also places a limit to its stringencies, and care is taken not to pile restrictions upon existing restrictions. The general tendency, though, is to show concern for every uncertainty. The basis for this approach is the Sages’ ruling that even a drop of ḥametz is forbidden; thus, if a mere crumb of ḥametz renders its entire mixture forbidden, so too individual halakhic opinions should be taken into account.
This is the root of the consistent difference between the rulings of Shulḥan Arukh, which follow general halakhic principles, and those of Rema, which account, le-khatḥila, for the stringent opinions. Nonetheless, in cases of pressing need Rema adopts the lenient approach, since halakha fundamentally accords with most poskim.
In general, Sephardim follow Shulḥan Arukh and Ashkenazim follow Rema. However, some Sephardic poskim tend to be stringent, and their rulings are accepted in some Sephardic communities.
. Some Sephardic poskim are stringent like Rema, as Kaf Ha-ḥayim states in 447:86, 88, and 119. Also, Zekhor Le-Avraham states at the beginning of the laws of Pesaḥ that the Sephardim have the practice to be stringent like Rema “to the extent that when it comes to Pesaḥ, we are Ashkenazim.” This is echoed by additional Sephardic poskim. On the other hand, in extenuating circumstances even Rema rules to be lenient in accordance with SA (in most cases).