From CNC.xyz Wiki
The Tool Offset G-Code, also known as G43 is used to retrieve stored tool length offsets that can be utilized during an Automatic Tool Change ATC operation using the M-Command M06. This command allows for changing the tool without having to re-zero the Z-offset.
Standard Format: G43 H00
Here the number after the H represents the desired tool number. H0 is a valid tool number. To clear the command and set a zero value offset, the number 0 can be used in place of the H-command.
In this example, Tool #5 will have its offset selected.
As a follow up example, Tool #2 will have its offset selected.
To clear all offsets and use zero offset, the following command is useful.
An alternative to clearing the tool offset is to use the G49 command to clear the offset.