Hello Alanndd,
I also had to get a site assessment and percolation test carried out.
If you organise this through a company offering a waste treatment system, they will undoubtedly push you to use their system, but you are not required to do so. They may offer a refund or discount on the test fee if you do of couse (always worth bargaining for anyway).
Each system has different requirements for percolation layout or area for distributing the treated waste liquid. So choosing a different system in the end may require you to recalc the percolation area, but the company you select should do this if you mention it before purchasing from them.
Your own engineer can do this independently also and as mentioned rejig the prec area later.
The local planners are interested in drainage suitability of the ground and whether you have a suitable area to build the percolation bed, while meeting all the requirements for minimum distance from houses, wells, roads etc.