Ian said that boron can form 3 covalent bonds once it has completed its lewis cycle. It has only 6 valence electrons. It requires more electrons, which it can only get by forming covalent (dative) bonds. It can do this in one of two ways, depending on the circumstances.
2. The way it occurs in AlCl3
This is the reason the both exist with stability.
Now, to answer your question,
BCl3 could fill it’s octet by internal or external means depending on the conditions, as it contains one smaller atom and one bigger atom.
Usually, the external method is preferred and so, it usually takes atoms from a lewis base and forms a coordinate bond.
Hope my handwriting was legible.