**Warning**: This is a mode selector that controls which period attributes are active!

**Valid Values**:
- `STATIC`: Fixed start/end dates (activates `start` and `end` date attributes)
- `DYNAMIC`: Relative to today (activates `startValueDynamic`, `startUnitDynamic`, `endValueDynamic`, `endUnitDynamic`) — most common
- `ATTRIBUTE`: Read from entity attributes (activates `startDateAttribute`, `endDateAttribute`)

**Note**: Only the period attributes corresponding to the selected mode are active. The others are hidden and ignored.