The Shulchan Aruch 168:6 writes that on פת הבאה בכסנין one makes a mezonos before and a bracha achas me'ein shalosh after. In Si'if 7 the Shulchan Aruch offers 3 possibilities as to the definition of פת הבאה בכסנין.
1. Bread with a sweet filling. 2. Bread where sweet items are kneaded into the dough (the Rama holds that unless a very large amount is kneaded in, it would maintain the status of bread, whereas the shulchan aruch considers is to leave the status of bread so long as one can taste the sweetness). 3. Bread made into thin, cracker-like wafers. The Shulchan Aruch paskens that the halacha is like all of the opinions. Meaning that if it meets any of the requirements it is considered pas ha'bah b'kisnin. The Biur Halacha (8) explains that the Shulchan Aruch paskened to make mezonos out of safeik, meaning that since it may qualify as פת הבאה בכסנין, we assume that it does and we don't make hamotzi and bentch. However, when it comes to considering something to be פת הבאה בכסנין that would require a bracha in middle of a meal, unless it meets all 3 requirements (such as a pie with a sweet crust), one would be lenient and not make a bracha out of safeik.
However, there is another school of thought cited in the previous Biur Halacha (v'halacha) in the name of the ma'mar mordechai that perhaps these are just different examples of פת הבאה בכסנין, but they all truly qualify. The Biur halacha says that based on this school of thought, any cake-like item that meets even one of the three requirements should need a bracha during the meal. The Biur Halacha himself assumes that most opinions agree to the first definition that when it contains a sweet filling it qualifies even if the bread is not sweet, but most don't agree to the second definition so standard cakes would still be at best a safeik and not require a bracha during the meal.
Hagaon Rav Yaakov Mi'Lisa, the Ba'al Chavos Da'as and Nesivos HaMishpat and derech hachaim, printed a small kunteros in the back of his hagaddah (ma'aseh nissim) on halachos of brachos. SEE HERE. He raises an amazing question. Birchas HaMazon is d'oraysa. How can we say that if one eats a filling amount of פת הבאה בכסנין that he should not say birchas hamazon, rather just make an על המחיה, since there is a safeik d'oraysa that he must bentch? The Ohr Sameiach suggests that perhaps the requirement to say the full birchas hamazon is not d'oraysa, therefore he can fulfill the d'oraysa with a ברכה אחת מעין שלש as well. This question compels R. Yaakov mi'lisa to take an approach similar to the ma'mar mordechai. The different opinions about what is פת הבאה בכסנין, are merely different examples of what can be considered פת הבאה בכסנין. The definition of פת הבאה בכסנין he derives from Rashi 41b d.h. pas. Rashi says that it is the type of bread that is made into shapes, mixed with spices and one generally eats only a modest amount of it. The definition of פת הבאה בכסנין is the type of bread that it is not the type of bread that one typically is ko'veiah their seudah on. Therefore, any bread which is used for some other function and not for קביעת סעודה would qualify as פת הבאה בכסנין and be mezonos. Based on this approach, any cake that one eats during a meal should require mezonos. Also, this enables us to include things like soft pretzels (superpretzel) which doesn't fit any specific category of פת הבאה בכסנין, into the realm of פת הבאה בכסנין and allow one to just make mezonos and ברכה אחת מעין שלש. Surely, even for פת הבאה בכסנין one would need to wash and bentch if they ate an amount that one typically is koveiah seudah on (furthermore, many poskim, including Rav Moshe (o.c. 3:32, 4:41) hold that other things that one eats combine to even a small amount of פת הבאה בכסנין to require one to bentch). However, so long as one eats a modest amount, they can consider even a soft pretzel as פת הבאה בכסנין based on r. ya'akov m'lisa.