This is the 12283rd most frequent Russian word.
испытательный
"Испытательный" means "trial," "test," or "probationary."
Here, 'испытательный' is used to describe 'срок', referring to a trial period to evaluate suitability for a job.
Я прошёл испытательный срок на работе.
I successfully completed the probation period at work.
In this context, 'испытательный' modifies 'полигон', denoting an area designated for testing, typically products or equipment.
Машина вышла на испытательный полигон.
The car went to the testing ground.
Here, 'испытательный' is used with 'этап', signifying a stage where trials are conducted to assess functionality or effectiveness.