Mishnah Nida 1.1:
שַׁמַּאי אוֹמֵר, כָּל הַנָּשִׁים דַּיָּן שְׁעָתָן. הִלֵּל אוֹמֵר, מִפְּקִידָה לִפְקִידָה, וַאֲפִלּוּ לְיָמִים הַרְבֵּה. וַחֲכָמִים אוֹמְרִים, לֹא כְדִבְרֵי זֶה וְלֹא כְדִבְרֵי זֶה, אֶלָּא מֵעֵת לְעֵת מְמַעֵט עַל יַד מִפְּקִידָה לִפְקִידָה, וּמִפְּקִידָה לִפְקִידָה מְמַעֶטֶת עַל יַד מֵעֵת לְעֵת. כָּל אִשָּׁה שֶׁיֶּשׁ לָהּ וֶסֶת, דַּיָּהּ שְׁעָתָהּ. וְהַמְשַׁמֶּשֶׁת בְּעִדִּים, הֲרֵי זוֹ כִפְקִידָה, וּמְמַעֶטֶת עַל יַד מֵעֵת לְעֵת וְעַל יַד מִפְּקִידָה לִפְקִידָה:
Shammai says: for all women it suffices [to reckon] their [period of uncleanness from their time [of discovering the flow]. Hillel ruled: [their period of uncleanness is to be reckoned retroactively] from the [previous] examination to the [last] examination, even if this was many days. The sages say: [the law is] not like the words of these or the words of those, but [the women are deemed to have been unclean] during [the preceding] twenty-four hours when this lessens the period from the [previous] examination to the [last] examination, and during the period from the [previous] examination to the [last] examination when this lessens the period of twenty-four hours. For any woman who has a regular period it suffices [to reckon her period of uncleanness from] the time she discovers the flow. And if a woman uses rags when she has marital intercourse, this is like an examination which lessens either the period of the [past] twenty-four hours or the period from the [previous] examination to the [last] examination. (Sefaria Translation)
The Mishnah uses the word פקידה to describe a checking. All the commentaries (including the Talmud on 4b) describe this act by way of the word בדיקה (checking). Meaning the verb פקד is never formally used to describe that same action. If so, why even use the word פקידה all together if all it is, is בדיקה.
