When a block is different for the server, than for the client, then its called a disguised block. For example a block is disguised, if its an iron block for the client, but a diamond block for the server. Technically ghost blocks and invisible blocks are also disguised blocks, where the block is an air block on either the client or the server side. Interestingly, a disguised block will transform into an invisible block, if its an instantmineable block clientside, but a non-instantmineable block serverside, and the block is instant-mined.