Edit: Because of the high number of downvotes on this question, I would appreciate if you could comment on why you think the question deserves so. I believe the question is clearly asking for the purpose of those vents. Suggestions and edits on how to make that clearer are much appreciated.
The servers are already stacked, and everything has been done as per the manufacturers instructions. I'm not asking if I'm doing it right, I'm asking why is that. Maybe the question belongs to another site on stack-exchange (I originally thought about the engineering site, since this is a engineering question, but assumed I would find the most expertise on this specific topic here).
We have a series of Dell rack servers and a few of them have vent holes on their top lid. This seems a bit odd since I would expect them to be designed for the purpose of being stacked. But if the units are meant to be stacked, the vent holes become useless.
I have found a similar question, Rack Mounting a Server with hot air vents on the top?, but the answers were focused on "do not worry about it". Well… then why are those hole vents there in the first place? Maybe something during production of the lids? Or to ease automatic assembly?
One of the comments in that question suggested reading the manufacturer's instruction. I found nothing on them.
Here's some pictures of the servers (well, pictures of the same model which I found on the internet):
Best Answer
(Some people are hard to please...)
These are likely system exhaust vents and fit into the total design of the product. Please understand that the system is designed and optimized for rack mounting in a proper enclosure with separation between hot and cold air aisle. These servers are designed to be placed in adjacent rack units, and there's a small gap that fits within the envelope of the rack unit standard.
That's the primary use case for this type of server and drives the product development target. When I say "total design", that may include factors like thermal management, vibration, fan speed, power consumption and acoustics.
If you see vents and don't understand why they may be there, possibilities could include eliminating known hot-spots or being part of an internal air circulation strategy for different potential configurations. There's already internal baffling and shrouding to help direct airflow between zones. These may simply be exhaust vents for air, heat or both. Maybe the server vibrates less with their presence, or perhaps the vents change the timbre of the fan sound.
Sometimes, you may see a design element meant for an edge case, like a fully-loaded PCIe riser cage or for a minimal configuration like a single CPU and single power supply. Or maybe it's relief for a situation where the server is sitting and running on a workbench or table. Perhaps this type of venting allows the server to run with a lower fan speed and rely more on passive heat dissipation. That can address both acoustic and power efficiency goals.
For instance, the rear-right edge of my HP ProLiant DL360p Gen8 1U rack mount server is the location of the 10GbE NIC module and the embedded RAID controller. There's also room for a PCIe card in the same spot.
My suggestion is to use the server as designed and indicated by the manufacturer. If you don't have a full rack and proper datacenter environment or will be operating in less-than-ideal conditions, make sure that the server's vents are unobstructed. Monitor your system's vitals, since this equipment is designed to record, report and react to its surroundings. Otherwise, I wouldn't worry too much about this.