If you have 300 GB of bandwidth, you will be able to upload 300GB to that VPS, or download 300GB from that VPS, or a mix between, like 100GB upload, 200GB download.
The OP has a point really, Most providers do not state what their transfer allocation is.
So like you mentioned its a mix, Most are 50/50
Example 300gb bw split into 150gb bw up/ 150 gb bw down.