There are two versions of SimpleMind for iPad & iPhone, SimpleMind and SimpleMind Pro.

SimpleMind Pro is the paid edition.
Simplemind, without in-app purchase, is the free edition. In-app purchase SimpleMind unlocks the Pro functionality, making it identical to SimpleMind Pro.
The only difference is Apple’s family sharing. This is not available in SimpleMind because Apple made in-app purchases not available for family sharing.