Salesforce has API request and concurrent request limits based on the type of Organization and the Salesforce Edition. After these requests limits have been exceeded, the following error is thrown: ConcurrentPerOrgLongTxn Limit exceeded, REQUEST_LIMIT_EXCEEDED with error code : 403. For most of the scenarios, Cloud Elements displays Provider Message and Error Code as-is which is received from the Vendor.
For more information on the request limits and different error codes we can refer to the below links:
https://developer.salesforce.com/docs/atlas.en-us.api_rest.meta/api_rest/errorcodes.htm
