Based on EIP-1155 on METADATA section:
The URI value allows for ID substitution by clients. If the string
{id}exists in any URI, clients MUST replace this with the actual token ID in hexadecimal form
what does 'CLIENT' mean here? is it the owner of the erc1155 contract or what? Can someone explain?
Hi, welcome to the community! 
I think just like the EIP-1155 description:
Example of such a URI: https://token-cdn-domain/{id}.json would be replaced with https://token-cdn-domain/000000000000000000000000000000000000000000000000000000000004cce0.json if the client is referring to token ID 314592/0x4CCE0.