Selecting any zone in the same region does not actually affect the function of ApsaraDB for Memcache.
Generally, when instances are in the same zone, the network latency is lower but the disaster recovery performance is weaker. When the applications require a lower internal latency, the application instance set can be deployed within the same zone.
Now, ApsaraDB for Memcache instances can be normally connected to ECS servers provided that the purchased ApsaraDB for Memcache instances belong to the same region as the ECS servers. You can purchase ApsaraDB for Memcache instances in the same zone as the ECS servers to achieve lower network latency.
For example, if you have an ECS server that belongs to Qingdao Zone B, you can purchase any ApsaraDB for Memcache instance in any zones in Qingdao and the instance can work properly. However, if you purchase instances in Qingdao Zone B, the network latency between the instances and the ECS server is lower.
Note: It is possible that resources are only available in one zone due to limited resource inventory in the data center, but this does not affect normal operation of your service.