@ OP
This is the thread to which you refer.
https://www.coh2.org/topic/3602/soviet-mines-vs-german-mines/page/1#post_id598247
I have uninvised your post, since the thread has been locked subsequently, and it may better help you to understand the following explanation.
As a general rule, we prefer that you do not necro old threads. We are not alone in this - it explains why you see so many images like this one on Google etc
http://imgur.com/a/4JP6h
I won't say all necro is bad, but most of it tends to be. I think many posters necro bcs they have been browsing, forget they are looking at old stuff and see something to which they want to respond, as if it were an active current thread, Your uninvised post is a good case in point.
You were responding to a post about mines, which no longer exist in the state they were in. e.g. The OP was partly moaning about a field of S barbed wire which no longer exists. To refer to this now could easily confuse newer members, who may not even know such a device ever existed.
The thread was started four years ago and there have been many changes to mines since then. Fortunately, the OP is still active on here, but the first person to reply to him on that thread is long gone - over two years ago.
As a result of your necro, a Strategist had written a reply before he realised you were necroing. That is easily done and it was a waste of his time, as you can see from his amended, irritated response.
You were responding to the OP by talking about Ostheer anti-personnel minefields with the warning boards - iirc, these did not even exist when that thread was started.
If you wanted to discuss the current state of mines for each side, it would have been preferable for you to start a fresh thread.
To finish, I will try to preempt your likely follow up: when does a thread become so old that to add to it is a necro? There is no hard and fast rule about this, but as a guideline, if there has been a patch, it is likely to render previous threads redundant.
tl:dr Never be afraid to PM a Moderator for guidance, if you are unsure about what may be a necro post.