What does it mean to have a soil improving effect under PFC 3?

Soil improver is quite a generic product category under the FPR. Broadly speaking if you comply with the safety and technical requirements of the Regulation you must just show that your product has the function: 

[...] of which is to maintain, improve or protect the physical or chemical properties, the structure or the biological activity of the soil to which it is added.

There is no further standard or data requirement for this. Thus trials (does not have to be according to CEN TS 17700)  or literature studies showing such an effect can be sufficient. If your product is already qualified as a soil improver in other countries this may also show such an effect, or if the product is for example a source of Corg + Nutrients, this is also often a soil improver. 

Surfactants may also under certain circumstances be a soil improver. 

In your technical documentation you would include such evidence.