When you hold Power Nexus, can you use the ability on the battlefield multiple times?

To answer your question regarding the Power Nexus battlefield: While you can trigger the ability on Power Nexus multiple times in a single turn, you can only successfully score once per battlefield per turn.

Ruling and Reasoning

According to FAQ #2852, Power Nexus starts a chain every time you hold it. Because the ability is a triggered ability ("When you hold here..."), it triggers each time the condition is met.

While Rule 447 states that a player may only "Score" once per battlefield per turn, the Power Nexus ability allows you to pay a cost to "score 1 point." Because this is a specific card effect that interacts with the scoring mechanic, it is subject to the limitation that you can only successfully score once per battlefield per turn.

Sequence of events:

1. Trigger: You move a unit to the Power Nexus battlefield, satisfying the "When you hold here" condition.
2. Chain: The Power Nexus triggered ability is placed on the chain.
3. Resolution: You have the option to pay the cost to score 1 point.
4. Subsequent Attempts: If you were to move your unit out and back into the Power Nexus battlefield (or move a different unit there), the ability would trigger again. However, because you have already scored at that battlefield for the turn, you would be unable to score a second point from that same battlefield, as per Rule 447 ("A player may only Score, from either method, once per Battlefield per turn").

Summary of Interaction

• Triggering: The ability triggers every time you hold the battlefield.
• Scoring Limitation: Per Rule 447, you are limited to scoring once per battlefield per turn. Therefore, while you can trigger the ability multiple times, you can only successfully complete the "score 1 point" instruction once per turn at that specific battlefield.

Note: I could not fully verify this ruling against the retrieved rules.