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Carefully read 38 CFR 3.156 a, b, and C. This is the regulation on reopened claims.
Understand that “reopening” the claim is not the same as awarding the benefit. YOu need to meet the threshold of “new and material evidence”. The evidence you send in needs to be both new and material, to reopen. “material” evidence means that the new evidence submitted has a “reasonable chance” of substantiating your claim.
Its a fairly low threshold, but it is not “no threshold”. Yes, VA can decide to reopen the claim, then deny it again.
It is always best to file the applicable nod within the year, and eliminate doubt. Its true, if you submitted new and material evidence it should be reopened and you should not have to file the nod. However, again, the VA can decide to reopen and then deny. So, you want to reopen the claim and file the NOD, both.
YOu see there are multiple reasons why VA can/does deny. One can be that evidence is missing. But there are other reasons also. Maybe you dont have a nexus, even with all your evidence in. In that case reopening wont help you, you still lack the nexus. But filing a nod may give you another chance to get a nexus and preserve your effective date, for example.