Final answer:
In the Dell XPS 9315 laptop model, the digit '9' represents the series of the laptop within the XPS product line.
Step-by-step explanation:
When servicing an XPS 9315 system board, the digit that represents the series of the laptop in the model’s name is 9. In the Dell nomenclature for the XPS line, the first digit after XPS indicates the series of the laptop, the second indicates the size of the screen, and the remaining digits often relate to the model version or revision. Therefore, in the model XPS 9315, 9 denotes the series, 3 indicates a 13-inch screen, and 15 signifies the specific model or version within that series.