Also, are there any general conventions that govern the usage of symbols in schematics?
Those look to me like testpoints. On an actual board they would either be empty pads or they may actually be testpoint components.
I'll hazard a guess that these are test points. These provide an convenient access to a signal. Test points are useful for debugging, quality control testing, calibration, troubleshooting. A signal can be probed at a test point. Or, a test signal may be injected into a test point to exercise the downstream circuitry.
On the PCB, a test point can manifest itself differently, depending on the expected type of test.
- It can be a throughole of surface-mount component for clipping a probe to.
- It can be throughole for sticking an oscilloscope into.
- It can be a surface-mount pad for automated test equipment.
As far as I know, there is no single standard schematic symbol for test points. Here are a few symbols that come with schematic software.
The standard designator for test point it
It's either a net label or a test point.
The test points could document locations for probing on a bed of nails functional/ICT fixture defined by a Test Engineer working with the Design Engineer for high fault coverage for fixture design.