When using one of the TDD modes that use GPS Sync (75/25, 50/50, 30/70), the time used for DL and UL transmissions is fixed. This means that for example, in 75/25 configuration, a maximum of 75% of the duration of the DL/UL cycle, will be used for DL and a maximum of 25% is used for UL. This ratio is fixed independently of the amount of traffic. On the other hand, flexible mode allows that DL/UL to be exactly that, "flexible" depending on the amount of traffic.