type BlockTag: BigNumberish | string;

A BlockTag specifies a specific block.

numeric value - specifies the block height, where the genesis block is block 0; many operations accept a negative value which indicates the block number should be deducted from the most recent block. A numeric value may be a number, bigint, or a decimal of hex string.

blockhash - specifies a specific block by its blockhash; this allows potentially orphaned blocks to be specifed, without ambiguity, but many backends do not support this for some operations.

Source

providers/provider.ts:42