It should be detailed in your Ops manual, but in our world:
Operational minima are those published on the approach plates.
Planning minima will be be those published minima plus company approved buffers (e.g. maybe a couple of hundred feet is added to any applicable cloud base minima, perhaps 500 metres to any visibility minima).
When planning for a sector the forecast minima for any alternates (ETOPS, other required en-route, and any destination alternates) need to be above planning minima.
Once you are en-route the planning minima is irrelevant and then the weather at the alternates simply needs to remain at or above the operational minima....
There will no doubt be national differences, specific rules concerning alternate minima etc...again Ops manual is the go to....