If an assignment of a patent requires "all rights in the undivided whole of the patent" to be transferred to another entity, what is meant by a "partial assignee"?

"Partial assignees" come up whenever there is joint inventorship.  Each joint inventor holds a separate, undivided interest whole in the patent, regardless of the level of their contributions.  Each joint inventor is therefore free to assign their rights to other individuals; but all parties having any portion of the ownership in the patent property must act together as a single composite entity when prosecuting patent matters before the United States Patent and Trademark Office.

The qualifying term "partial" in the expression "partial assignee" therefore does not refer to the fact that "less than all of the rights in the patent were transferred to the partrial assignee." As explained in the last post, assignments of a patent require the rights holder to transfer all of their rights in the undivided whole of the patent.  Anything less than a transfer of all of the patent rights would be considered a "license" rather than an assignment.

Rather, the qualifying term "partial" as used in "partial assignee" means that a joint inventor assigned his full interest in the undivided whole of the patent to another entity.  There is at least one other party in the world whose participation is necessary for there to be a single composite entity.  For example, if some invention was invented by joint inventor A and joint inventor B, and later joint inventor A gets terminally sick and executes an agreement to to transfer all rights he holds in the patent to his son, then the son would be considered a "partial assignee."   That is to say, it is "partial" in relation to the single composite entity, rather than "partial" with respect to the number of rights transferred.