What does "maximum update interval" mean for Mode S GICB registers? Transponders have registers for Mode S data and configuration. These registers are updated periodically to provide Mode S services such as ADS-B Airborne Position squitter.

DO-260B and similar documents have Register Allocation Tables that states maximum update intervals for each register(if applicable).

But there is also a register timeout mechanism that states "if a register can not be updated within twice the maximum update interval, it shall be zeroized". 

So, what happens if certain register is updated at every (1.5 * maxUpdateInterval) ? Are these maximum update times mere suggestions or is there another rule that forces us to update registers at most maximum update interval?
If you update the register, the values will be set to the values provided.
From that moment, a timer starts running. When the timer reaches 2 * maxUpdateInterval, the register will be set to all zero. 

If a radar extracts the register before the 2 * maxUpdateInterval is reached, it will get the values provided at the update. If it will extract the register after 2 * maxUpdateInterval is reached, it will get all zeros in the reply. 

For ADS-B, transmission of the register is inhibited (IIRC) when the register is set to all zeros.