Sub-strategy

Yes — a sub-strategy can be kept aside: switched off, still inside the Combined, and left out of every backtest until you switch it back on. On the Combined's Sub-strategies card each sub-strategy in use has a pause button ("Keep … aside") beside its remove ×. A kept-aside sub-strategy moves to a group under the table titled "Kept aside (n) — left out of the simulation", with a play button ("Switch … back on") and the same remove ×.

• Nothing is lost while it is kept aside. Its own settings and its weight at the Combined level stay exactly as they were, and switching it back on brings both back unchanged. Removing it, by contrast, takes it out of the Combined.
• It is left out of the run. The Combined's capital is split across the sub-strategies in use only; the table on the card lists those, and its weight total covers those alone.
• Its own page says so. Opening a sub-strategy kept aside shows "This sub-strategy is kept aside: it stays in the Combined but is left out of the simulation." with Switch back on, which follows the same plan limit as the play button.

• It does not count toward your plan's limit on sub-strategies in use in a Combined — only the ones switched on do, and a Combined always counts as at least two. Its own settings (positions, attached screeners, risk conditions, allocation method) are still held to the plan; see plan compliance.
• At the plan's limit it cannot be switched back on. Pressing play leaves the sub-strategy kept aside, and the card says "The Advanced plan allows 10 sub-strategies in use. To switch another one back on, keep one aside first, or switch to Ultimate." — naming your own plan, its limit and the first plan that allows the number you asked for, or leaving the last clause out when no plan does. A Combined saved with more sub-strategies in use than your plan allows — one carried over from the previous platform, say — can still switch sub-strategies back on up to the number it was last saved with; only going beyond that is refused.
• On a plan without the Combined nothing can be switched back on. A Combined always counts as at least two, so on a plan allowing one sub-strategy in use the card says "The Starter plan does not include the Combined. To use one, switch to Advanced." instead.

• Adding a sub-strategy counts too. A strategy you add joins in use, so at the plan's limit choosing one to add leaves the roster as it is, and the card says "The Advanced plan allows 10 sub-strategies in use. To add another one, keep one aside first, or switch to Ultimate." The Add a strategy window opened from the + beside the Combined's tabs follows the same rule: choosing a strategy or Create a blank strategy there adds nothing and shows the same sentence.
• A Combined still needs two sub-strategies in use to run. With fewer switched on, Backtest is refused with "Running this needs 2 sub-strategies switched on."
• Kept aside or in use, a Combined holds at most 50 sub-strategies. At 50, choosing one more to add leaves the roster as it is, and the card says "A Combined holds at most 50 sub-strategies, in use or kept aside."

You build a momentum strategy and run it on its own. You then add it to a Combined alongside two other strategies, and the Combined takes a snapshot copy of the momentum strategy exactly as it stood at that moment — now one of its three sub-strategies. A week later you change the standalone strategy's rebalance cadence. The sub-strategy inside the Combined keeps the old cadence — it was frozen at add time — so the standalone and the sub-strategy now backtest differently.