How is data used on post paid? Telkom mobile vs all network data

Hi Guys

How does telkom decide on which data bundle to use first on their post paid products? I get 1.1GB of all network data and I am thinking about getting a 25GB+25GB for R999 telkom mobile data.

Will telkom only use the 25GB package if on the telkom network and then when not in range switch to the 1.1GB? Or are they dodgy and use up your all network data first?

Thanks for the help :)