Namespace prefix for managed packages in salesforce

Namespace prefixes are used in managed Force.com AppExchange packages to differentiate custom object and field names from those in use by other organizations. After a developer registers a globally unique namespace prefix and registers it with AppExchange registry, external references to custom object and field names in the developer's managed packages take on the following long format:

namespace_prefix__obj_or_field_name__c

Setup -> Build -> Create -> Packages -> Developer Setting -> Edit

Before giving namespace prefix create your packages.

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