Same-Day Substitution

An offsetting change in a margin account, made over the trading day, that results in no overall change in the value of the account. When a same-day substitution is made, a margin call is not generated.

Investopedia Says:
A same-day substitution happens when a rise in the market value of one margin security is offset by an equal decline in another.

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Find out what margin is, how margin calls work, the advantages of leverage and why using margin can be risky. Margin Trading

Offsetting changes in a Margin Account in the course of one day, resulting in neither a Margin Call nor a credit to the Special Miscellaneous Account. Examples: a purchase and a sale of equal value; a decline in the Market Value of some margin securities offset by an equal rise in the market value of others.