Covering the line vs the parry vs the block - All kinda the same thing, yet all kinda different.
- Covering the line does not engage the opponent's blade, it's more of a 'I see you and want you to know I have'. It forces the opponent to look for other options.
- Parrying engages the blade, deflecting the weapon away from it's target. It can be used as a way to enter, a bridge if you like, to create an offensive opportunity.
- Blocking OTOH often happens because you are late, and the only way not to get hit is to use your blade to protect yourself (There is a difference here between edged and point only weapons, here I'm mostly talking about slicing/cutting blades). It too can be used as an opportunity, but often, because you are so 'late', it just buys time. It is possibly the least beneficial move offensively, but the most useful defensively, especially on an exit.
The other option of course it to open the line, to invite the opponent in.
All are range/time and angle dependent, and require a recognition of what is real threat vs what is not.
Dueling by it's nature, plays at the boundary between danger and safety - the opponent will not be tempted if it does not look ... tempting, and you cannot reach them without getting into range, which means you are either in danger, or have to 'seem' in danger.
A useful practice is to gain insight into how close you can play the margin, how little you need to engage with the opponent's blade, and how late you can be if you have to defend yourself.
Ideally you only need to use your weapon to protect yourself after you have struck them.
Less risky, is to engage on the way in to create a safer path, making sure the opponent cannot counter.
At worse, you misread and scramble for a block .... Unless of course you are really tricky and only seem like you are late ... using awkward and 'disadvantageous' positions to really surprise your opponent off a late block.
Riskier, but high percentage, and really funny if you can pull it off.
Bottom line - Don't waste your blade engaging the opponent's blade unless you are going to use it for good.
And practice what you can get away with.
How late? How close?