# Precedence for zonal shifts
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There can be no more than one applied zonal shift at a given time. That is, only one practice run zonal shift, customer-initiated zonal shift, autoshift, or Amazon FIS experiment for the resource. When a second zonal shift is started, ARC follows a precedence to determine which zonal shift type is in effect for a resource. 

The general principle for precedence is that zonal shifts that you start as a customer take precedence over other shift types. However, be aware that a currently-running Amazon-initiated practice run prevents you from starting an on-demand practice run.

To illustrate precedence in ARC, the following is how precedence works for example scenarios:


| Zonal shift type applied | Zonal shift type initiated | Result | 
| --- | --- | --- | 
| Amazon FIS experiment | Practice run | The practice run will fail to start, as the Amazon FIS experiment takes precedence.  | 
| Amazon FIS experiment | Manual zonal shift | The Amazon FIS experiment will be canceled, and the manual zonal shift will be applied.  | 
| Amazon FIS experiment | Zonal autoshift | The Amazon FIS experiment will be canceled, and the zonal autoshift will be applied.  | 
| Amazon FIS experiment | Amazon FIS experiment | The initiated Amazon FIS experiment will fail to start because there is an existing experiment running that triggered the Amazon FIS autoshift action. | 
| Practice run | Manual zonal shift | The practice run will be canceled and the outcome set to INTERRUPTED, and the zonal shift will be applied. | 
| Practice run | Amazon FIS experiment | The practice run will be canceled and the outcome set to INTERRUPTED, and the Amazon FIS experiment will be applied. | 
| Practice run | Zonal autoshift | The practice run will be canceled and the outcome set to INTERRUPTED, and the zonal autoshift will be applied. | 
| Manual zonal shift | Practice run | The practice run will fail to start. | 
| Manual zonal shift | Amazon FIS experiment | The Amazon FIS experiment will fail to start, or fail if it's already in progress. | 
| Manual zonal shift | Zonal autoshift | The zonal autoshift will be ACTIVE but not APPLIED on the resource. The manual zonal shift takes precedence. | 
| Zonal autoshift  | Amazon FIS experiment | The Amazon FIS experiment will fail to start, or will fail if it's in progress. | 
| Zonal autoshift  | Manual zonal shift | The zonal autoshift will be ACTIVE but not APPLIED on the resource. The manual zonal shift takes precedence. | 
| Zonal autoshift  | Practice run | The practice run will fail to start, as the zonal autoshift takes precedence. | 

The traffic shift that is currently in effect for the resource has an applied zonal shift status set to `APPLIED`. Only one shift is set to `APPLIED` at any time. Other shifts that are in progress are set to `NOT_APPLIED`, but remain with `ACTIVE` status.