Transferability
As you may know, all Sablier streams are represented by NFTs. The NFT representing a stream is owned by the stream recipient. Whoever owns the stream NFT becomes the stream recipient.
When creating the stream, users have the ability to set it as transferable or untransferable.
If transferable, recipients have the ability to transfer the NFT (meaning the stream) to another wallet. They have, by extension, also the ability to, for example, sell it on an NFT marketplace, or borrow against it by using it as collateral in an NFT lending protocol.
If untransferable, recipients do not have the ability to transfer the NFT (meaning the stream) to another wallet, and by extension, do not have the ability to sell it on an NFT marketplace or borrow against it in an NFT lending protocol.
If you are using Sablier for vesting, and do not want your investors and/or employees to be able to exit their position without the vesting being complete, you should set up the streams as untransferable.