NOR Flash Memory Full chip vs Block vs Sector Erase
First thing to understand about NOR flash is that programming individual bytes can only change 1-bits to zero but cannot modify a bit already set to zero. To set a bit back to one, the entire smallest-erasable-section must be erased.

In this chip, the smallest erasable section is a 4Kbyte sector (4096 bytes, for example bytes 0-4095). The 32Kbyte (32,768 byte) and 64Kbyte (65,536 byte) block are simple groups of sectors that can be erased as a single operation. It doesn't matter if you erase a sector, either size block, or the entire chip. All bytes in the range will be set back to all ones (0b11111111).

In most NOR chips, it is not actually required to erase a byte before programming. However, such a re-programming again, can only change ones to zeros. Writing all zeros to an already programmed non-zero byte or bytes can be useful for thing like quickly marking a block of data as invalid or obsolete.