IF you are asking about "sealing" in context of "pens" - CA in general is a great sealer. Use thin on the first couple of coats and then medium or thick after. Shelac is a good sealer as a prefinish for other finishes.
Lacquer is a good sealer and dries fast but sometimes Shelac under it does well.
If using CA as a finish, do not use other oils on oily wood before the finish or the chances for the finish "lifting" or separating is increased considerably.
Sealing is also recommended for some woods to prevent color bleed when segmenting. Lacquer can cause bloodwood to bleed over onto other woods, and in the case of holly, is very noticeable. DAMHIKT!
Some black woods will also bleed onto lighter woods if segmenting. CA coatings as in sealing, will prevent the bleed overs.
Another thing to consider - you don't see the normal wood finish terminology used as much here very much. The wood blanks on pens are wood, but are used TOTALLY different from furniture and most flatwood, and even large wood turnings. They (pens) are examined extremely close because the pens are smaller than most wood objects. Flaws in the finish are more quickly noticed on pens. Pens are held by oily grimy hands and often placed in humid shirt pockets that furniture or bowls are not. They need different finishes to protect it than most wood objects - but they don't have to have it. Many people still want to finish a pen like they would normal wood objects, and that is fine. Just remember, you don't put normal wood objects in a humid shirt pocket or a hot car and then take into an air conditioned room. Pens need something different OR they need to be taken care of much more often than a wood bowl or chair or other.