Under the signature, there's a partial value of the long random ID (unique for each signer) and it is called Signer ID. It is a unique ID for each signer and can be presented to prove a unique identity being maintained in case of legal issue.
However, there are cases you may like to hide the signer ID when you try to sign the PDF document. You may access Foxit eSign Cloud online portal, then go to Settings > Sending/Signing >uncheck Signer ID under Signature Information to Include > Save. After done, the newly added signature will not include Signer ID.