diff --git a/routes/sources.js b/routes/sources.js
index 9500104..67eefcd 100644
--- a/routes/sources.js
+++ b/routes/sources.js
@@ -87,13 +87,23 @@ module.exports = function(pool) {
         if (!Array.isArray(cols)) {
             return res.status(400).json({ error: 'body must be an array' });
         }
+        // Exactly one per source: the scope is a single IN list against a single
+        // column, and two flagged would silently mean whichever one a .find()
+        // reached first -- the trap is_key already fell into (see CLAUDE.md).
+        const territoryCols = cols.filter(c => c.is_territory).map(c => c.cname);
+        if (territoryCols.length > 1) {
+            return res.status(400).json({
+                error: `Only one column can be the territory. Flagged: ${territoryCols.join(', ')}`
+            });
+        }
+
         const client = await pool.connect();
         try {
             await client.query('BEGIN');
             for (const col of cols) {
                 await client.query(`
-                    INSERT INTO pf.col_meta (source_id, cname, label, role, is_key, dim_group, dim_period_col, in_grain, opos)
-                    VALUES ($1, $2, $3, $4, $5, $6, $7, $8, $9)
+                    INSERT INTO pf.col_meta (source_id, cname, label, role, is_key, dim_group, dim_period_col, in_grain, is_territory, opos)
+                    VALUES ($1, $2, $3, $4, $5, $6, $7, $8, $9, $10)
                     ON CONFLICT (source_id, cname) DO UPDATE SET
                         label          = EXCLUDED.label,
                         role           = EXCLUDED.role,
@@ -101,6 +111,7 @@ module.exports = function(pool) {
                         dim_group      = EXCLUDED.dim_group,
                         dim_period_col = EXCLUDED.dim_period_col,
                         in_grain       = EXCLUDED.in_grain,
+                        is_territory   = EXCLUDED.is_territory,
                         opos           = EXCLUDED.opos
                 `, [
                     sourceId,
@@ -111,6 +122,7 @@ module.exports = function(pool) {
                     col.dim_group      || null,
                     col.dim_period_col || null,
                     col.in_grain       || false,
+                    col.is_territory   || false,
                     col.opos           || null
                 ]);
             }
diff --git a/ui/src/views/Setup.jsx b/ui/src/views/Setup.jsx
index 09f1ed1..8b137ac 100644
--- a/ui/src/views/Setup.jsx
+++ b/ui/src/views/Setup.jsx
@@ -354,6 +354,7 @@ export default function Setup({ refreshSources }) {
                       <th className="px-3 py-1.5 font-medium">role</th>
                       <th className="px-3 py-1.5 font-medium text-center">key</th>
                       <th className="px-3 py-1.5 font-medium text-center" title="Include this column in the display grain — the load is pre-aggregated to the flagged columns">grain</th>
+                      <th className="px-3 py-1.5 font-medium text-center" title="The column an account's territory is expressed in. Accounts see and change only rows whose value here is on their list. One per source.">territory</th>
                       <th className="px-3 py-1.5 font-medium">group</th>
                       <th className="px-3 py-1.5 font-medium">period col</th>
                       <th className="px-3 py-1.5 font-medium">label</th>
@@ -390,6 +391,25 @@ export default function Setup({ refreshSources }) {
                             className="cursor-pointer disabled:opacity-20"
                           />
                         </td>
+                        {/* Radio, not a checkbox: exactly one column per source,
+                            and the shape of the control should say so rather than
+                            leaving it to a save-time error. */}
+                        <td className="px-3 py-1.5 text-center">
+                          <input
+                            type="radio"
+                            name="pf-territory-col"
+                            checked={!!col.is_territory}
+                            onChange={() => {}}
+                            onClick={() => setEditedCols(prev => {
+                              // clicking the chosen one again clears it, since a
+                              // radio otherwise has no way back to "no territory"
+                              const already = !!prev[i].is_territory
+                              return prev.map((c, x) => ({ ...c, is_territory: !already && x === i }))
+                            })}
+                            disabled={col.role !== 'dimension'}
+                            className="cursor-pointer disabled:opacity-20"
+                          />
+                        </td>
                         <td className="px-3 py-1.5">
                           <input
                             type="text"