I am guessing it is this (assuming that the seminary qualifies as an "agency other than a district"):
2.12.1.5 An individual member of the Synod who is serving an agency other than a district and other than a missionary or chaplain serving under call by the Synod shall hold membership through the district designated by that person if approved by both the president of that district and the president of the district in which the agency is located, but shall be subject to the ecclesiastical supervision of the president of the geographical district in which the agency is located. When all voting members of the agency are members of a non-geographical district, membership shall be held through that district.
The 2019 convention actually amended this provision so that it now reads as follows:
2.12.1.5 An individual member of the Synod serving in any other position shall hold membership through the geographical district in which the place of service is located, unless serving an agency or mission of a non-geographical district, in which case membership shall be held through that district.
Presumably this means that Synod members serving at Concordia Seminary must now be members of the Missouri District, and Synod members serving at Concordia Theological Seminary must now be members of the Indiana District. This change may have been prompted by the Matthew Becker situation--he was on the faculty at Valparaiso University in Indiana, but still a member of the Northwest District from his time at Concordia University Portland.
WHY a man serving in Missouri would seek to be a member of Atlantic District is another question, though....
Especially since, even under the previous version, his ecclesiastical supervisor was the Missouri District President regardless. From that standpoint, what did it even mean to be a "member" of another district?
In other words, prior to the 2019 convention, it was possible for a professor to have membership in a "home" district, the district from which he came. Which answers the prior question.
The way it gets played church-politically is that somebody's trying to pull a fast one. In T. Scholl's case, he was at home in the Atlantic District. That's all there was to it. This is my recollection from five years ago when last I was in the supervisory role.
An issue for ecclesiastical supervision is that the President of the Missouri District now is saddled with supervision of way over a thousand clergy, when you count in all the emeritus who are still there plus the seminary faculty and the many congregations. Absolutely non-manageable, especially given the percentage of hard and consumptive cases like restriction, suspension and conflict/clergy burnout and the like.
For example, another thing I remember from the old days is that when you leave the roster of the LCMS and want to get back on it, you have to go through the office of the district president of the district you left from. This could be many years and many district presidents down the line. That happened to me a few times, and was a) weird b) hard because of the amount of digging through old records required c) difficult because of the discernment involved in dealing with whatever the politics had been vs. what they now were to get down to the facts of the case. So - imagine that if, as the bylaw now requires, you have 1500 clergy (whatever the number is exactly) under management, as in the Missouri District. On second thought, don't imagine it. It's just been created; give the Missouri District President time to get ready for that nervous breakdown.
Dave Benke