If you are familiar with this stuff then you probably don't need the information I have added. So let me start with the question:
Can you prove that category $\mathbf{Lat}$ is not a full subcategory of category $\mathbf{Pos}$?
I dived into the theory of universal algebras and encountered lattices. I noticed that a lattice $\left(L,\wedge,\vee\right)$ can be desribed as a poset that is equipped with binary products and coproducts. Here $a\leq b$ is defined by $a=a\wedge b$. Then $a\sqcap b=a\wedge b$ and $a\sqcup b=a\vee b$. There is a category $\mathbf{Lat}$ having lattices as objects and functions that respect $\wedge$ and $\vee$ as arrows. These functions are order-preserving which makes them arrows in $\mathbf{Pos}$ having posets as objects and orderpreserving functions as arrows. So $\mathbf{Lat}$ is a subcategory of $\mathbf{Pos}$ but I 'know' that it is not a full subcategory. As a confirmation of that I am looking for an arrow in $\hom_{\mathbf{Pos}}\left(L,L'\right)$ that is not an arrow in $\hom_{\mathbf{Lat}}\left(L,L'\right)$. Here $L$ and $L'$ are lattices. So it is about a function that is orderpreserving, but does not respect (at least one of) the operations $\wedge,\vee$.