I was in the same situation before and sent a message to VETASSESS to clarify my work experience that would be considered as it is stated on the VetAssess Outcome letter or in my employer's letter for further stages of migration process
And this is the response from them
"Dear Tuan,
Thank you for your email. In addition to your qualification (Master of Science) meeting the educational requirements for the nominated occupation, VETASSESS only required to consider at least one year of post-qualification highly relevant employment at an appropriate skill level within the last five years for the nominated occupation. Therefore the two most recent contractual periods of employment with BioCSL have been considered as meeting the minimum one year employment skills assessment criteria. The period of employment listed on the outcome letter was the duration of employment that has been considered for assessment.
I hope this information is of some help to you. Please contact us again if we can be of further assistance.
Regards,"
So in my opinion, they just pick whatever in our files (qualification, recent employment contracts) those meet the requirements of skill assessment. I hope this helps some how,
Regards,