Is there a reference of what var's are used for what?
We've the below trigger documentation that can at least give you an idea what variables are available to a trigger.
    www.joomlapolis.com/documentation/279-community-builder/tutorials/18358-using-cb-triggers
Once we've rewrote how we handle triggers we'll be improving our phpdoc for them then be able to parse out that phpdoc into documentation to significantly improve the information available for each trigger, but that won't be for awhile. There's no harm in asking on the forums though as we'd be glad to explain.
How do I find out what to use if I need more cbsubs info?
For that trigger specifically the variables are as follows.
Code:
/**
 * Called at each change of user subscription state due to a plan activation or deactivation
 *
 * @param  UserTable        $user               The user owning the $subscription with that $planId
 * @param  string           $status             New status: 'A'=Active, 'X'=Expired, 'C'=Cancelled
 * @param  int              $planId             Plan Id which is changing status
 * @param  int              $replacedPlanId     Replaced Plan Id in case of an upgrade
 * @param  ParamsInterface  $integrationParams  Integration parameters for that plan $planId
 * @param  string           $cause              'PaidSubscription' (first activation only), 'SubscriptionActivated' (renewals, cancellation reversals), 'SubscriptionDeactivated', 'Denied'
 * @param  string           $reason             'N' new subscription, 'R' renewal, 'U'=update )
 * @param  int              $now                Unix time
 * @param  cbpaidSomething  $subscription       Subscription/Donation/Merchandise record
 * @param  int              $autorenewed        0: not auto-renewing (manually renewed), 1: automatically renewed (if $reason == 'R')
 * @return void
 */
public function onCPayUserStateChange( $user, $status, $planId, $replacedPlanId, $integrationParams, $cause, $reason, $now, $subscription, $autorenewed )
 
So for example var1 is the $user object. To get the users user id from that variable you'd just use [var1_user_id] (but typically you're fine just using [user_id] since that variable is detected as the User for that auto action).