I rarely got to know the actual deployment scale of anything I've done. Let's make a list:
Ground software for an observation satellite. My internship was about implementing a dead simple neural "network" (2 hidden layers, no feedback), everything was specified from up top, we didn't even get to touch the learning algorithms. Impact? I guess a big flat zero, since all the differentiators was in the learning parameters.
Peer-to-peer social network before Facebook. Never made a cent.
Geographic Information System for the military. I was for obvious reasons not allowed to know enough to estimate the impact of my work. And even then all decisions was made by the customer, and once the user (a different entity) saw the Rube Goldberg contraption we dully made for them they predictably balked, and we did what we could from there. Which was, not that much. I did some useful stuff for sure, but mostly I participated in a system that was arguably worse than the one that preceded it.
A visualiser for civil radar data. Data in, little planes in the screen out. And other nice stuff. I designed a simple C++ API that allowed the client to write business code faster than we would have ourselves (if only because of communication overhead), saving weeks of work. That contribution was utterly ignored for personal reasons, and I was eventually out. I have no idea what my actual impact was, because I don't know how far the project even went, and how widely it was eventually deployed.
The maintenance of ground software for small civil observation drones. I did some cool stuff, but then was asked to transfer ownership of this software to a recently bought team (that did stuff similar to the company I worked for). I could have known how many drones were actually deployed, but to be honest my thing just saved a few minutes of flight, while most of the cost is to get the drone and its operator on site. That company was never really profitable, I hope the good people I met there are doing well.
Scripting language for a programmable logic controller test environment. For the military, so I don't think I was allowed to even know the size of the team we'd deliver the software to. I got good feedback from them (they were happy about what I did), and I'm pretty sure my static typing made things easier for them than if I had just picked Lua or something, but how easier, and how much money it will save in the long run I have no freaking clue.
Stuff in a missile company I cannot disclose. I believe my impact was almost nil, I couldn't stand their abysmal tech environment.
Maintenance of a ground troops training system (a glorified laser tag, with debrief helpers and so on). Good luck assessing the impact of a better system, and I was just asked to do small tasks I could barely chose anyway.
Prototype ADAS system. It was never deployed. Actual impact was therefore basically nil. Cool stuff to work on though, the CAN bus is a think of beauty. One of the rare instances where I could actually learn from example, instead of seeing yet again one of the gazillion obvious ways how not to do stuff.
Ground software for some IoT device. Impact fundamentally uncertain, we had yet to sell it to anyone.
Incident reporting software, based upon a more generic distributed base. I made the encryption layer (between users & company server), with a security based on PAKE (thus avoiding a PKI, which simplified the work of the sysadmin, at a slight loss of security). Impact fundamentally uncertain, we had yet to sell it to anyone.
Charging stations for electric vehicles. I did the TPM provisioning, and mentioned a low-key security issue along the way. I participated in a questionable micro-service that was meant to help user interfaces (yeah, their IoT stuff had a micro-service architecture). Impact: whatever I did didn't save them: one year after I left, they're now going under.
Preliminary study on the possible use of AMD-SEV to prevent users from peeking at our secret sauce (DRM). I don't think I was allowed to know the list of clients, and it's not even the only alternative. I don't think I could ever have assessed the long term impact of my work there.
Flight recorder for trains (not a flight recorder then, but you get the idea). I just did little tasks here and there, didn't get the chance to have a good bird's eye view of the thing or its environment. Deployment base was knowable, but the business impact of my work was likely minimal, beyond "finish this step so we can show the client we're on track for the next short term milestone". The whole thing is a heap of technical debt, common components are impossible to update (user projects aren't locked to a given revision, they all pull from trunk), the build system is a home made monstrosity that doesn't help more than the standard monstrosities (I hate build systems)… and I was just axed from a round of layoffs.
Cryptographic library I did on my free time: https://monocypher.org/ Nice little thing with a significant user base in the embedded ecosystem (not even my primary target). I controlled everything from start to finish, and I have no idea how many users I have, let alone how much time and money I saved them. In part because it is so simple, with such an outstanding documentation (which I mostly didn't write), that most users don't even have to bug me.
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To sum this up, my resume looks fairly horrible with respect to what I know of my actual business impact. Most of it, I think, was entirely outside my control. And I don't think I'm exceptional in this.