Lower than standard Takeoff Minima
Your should find C057 of your Ops Specs IFR Takeoff Minima Part 135 (we don't care about Part 91 because they can do anything foolish).. states (it's a standard verbiage the FAA uses when creating the document):
Standard takeoff minima = 1sm or 5000 RVR for 2 engines or less.
a. When a takeoff minimum is not published, the certificate holder may use the applicable standard takeoff minimum (the chart) and any lower than standard takeoff minima authorised by these operations specifications. (Remember, not all Part 135 operators are approved for lower than standard minima) When standard takeoff minima or greater are used, the RVR report is controlling.
When published takeoff minima is greater than the applicable standard takeoff minima and an alternative procedure (such as a minimum climb gradient etc) is NOT prescribed, then the certificate holder shall not use a takeoff minima lower than the published. (i.e. the published is controlling for a reason - such as terrain, obstacles etc - that's why is higher than standard)
So your airline might allow in the Ops Specs for you to do a 1200 RVR takeoff. But if the runway you are taking off on, has no climb gradient or no specified obstacle clearance the chart may list minimums non-standard, i.e. 1 1/2 sm for takeoff, then the non-standard is controlling and you cannot use your Ops Specs lower-than standard minimum because for some reason, higher visibility is mandatory.
You can pick any chart that has a takeoff visibility of greater than 1sm as an example.