Anyone please explain me, what is meant by fixed bins in WM?
Fixed bins are defined on storage type level and per material number in a material master record (WM tab 2). You define bin number, and several limiting quantities (min, max, replenish qty).
In practice, they are usually positioned practically at the lower level of the warehouseso that the picker can take material from these bins. Generally they are used for picking small amounts from pallets in [pick-pack-deliver] process. This way picker knows that material A will always be available in bin X, and you can also instruct the picker via mobile terminal to bin X in order to pick material A.
Such picking process can be used for delivery to the customer or for replenishment of own production process.
Fixed bins are associated with the standard SAP method for replenishment. They are periodically replenished from the main warehouse - using trans. LP21 manually or in a job run. When they get empty, the system will trigger replenishment and create TO.
Practical process, worth considering.
Hopefully this could help a bit.