options : ussr / usa neutral : if checked, does this mean neither nation will declare war on axis powers ? thanks

Yes that is right

Neutral means they will only enter the war if the German player declares war on them.
Historical for the USA will mean they enter the war in December 1941
Random means they will enter automatically once their rating reaches 100% (this can be seen on the war map)

If the USA is set to Random the % will go up by about 4% a turn, it will also go up by a % every time the Axis player declares war on a neutral country. (it will also go down if the Allied player declares war on a country).

If Russia is set to random it will go up by about 6% a turn.