The HIGH operator is HIGH.
High Byte: HIGH(<operand>)
This operator returns the high byte of the address of a memory location.
Assume data1 is a word located at address $1050 in the memory.
LDA #HIGH(data1)
This instruction will load the immediate value of the high byte of the address of data1 ( $10) in register A.
LDA HIGH(data1)
This instruction will load the direct value at memory location of the higher byte of the address of data1 (i.e., the value in memory location $10) in register A.