The "Triple Caster System" at the rear of the machine has a grease zerk that you want to make sure gets done weekly. With the constant turning, under weight, they will wear out and become loose. This is considered a "wear" part so eventually with excess use it will need to be replaced.
Likewise, when you stand the machine up, there will be 4 bearings that should be greased weekly. These not only support the differential but maintain smooth turning and movement. These are also considered a wear part but typically last for the life of the machine as they aren't turning at 1000's of rpms.