The word אך indicates a limitation in the second sentence; it means to say in the sentence where the word אך comes. And nevertheless, it is different than the word רק, which excludes a matter from the previous sentence.

Deuteronomy 12:15-16 says,רק בכל אות נפשך תזבח ואכלת בשר כברכת יהוה אלהיך אשר נתן לך בכל שעריך הטמא והטהור יאכלנו כצבי וכאיל. רק הדם לא תאכלו על הארץ תשפכנו כמים(Notwithstanding you may kill and eat flesh within all your gates, after all the desire of your soul, according to the blessing of the LORD your God which He has given you; the unclean and the clean may eat thereof, as of the gazelle, and as of the hart. Only you shall not eat the blood), using the wordרקto mean “only”. Here, blood is excluded from the previous sentence as being permissible to eat.

Leviticus 11:3-4 says,כל מפרסת פרסה ושסעת שסע פרסת מעלת גרה בבהמה אתה תאכלו. אך את זה לא תאכלו ממעלי הגרה וממפריסי הפרסה את הגמל כי מעלה גרה הוא ופרסה איננו מפריס טמא הוא לכם(Whatsoever parts the hoof, and is wholly cloven-footed, and chews the cud, among the beasts, that may you eat. Nevertheless these shall you not eat of them that only chew the cud, or of them that only part the hoof: the camel, because he chews the cud but parts not the hoof, he is unclean to you), using the wordאךto mean “nevertheless”, which is used to teach that one cannot eat of any type of animal which does not satisfy all of the criteria listed in the first sentence. The camel does not satisfy all of these criteria, since it does not have split hooves, even though it chews its cud⁷⁶⁸.

footnotes: ⁷⁶⁸ Malbim, HaTorah v’HaMitzvah Shemini 68.