Cell group a bearer's RLC bearer belongs to (TS 38.331 RLC-BearerConfig): the master cell group is the one the UE is anchored on, the secondary cell group the one a dual-connectivity secondary node serves. A bearer with both is a split bearer.

Source code

/// Cell group a bearer's RLC bearer belongs to (TS 38.331 RLC-BearerConfig): the
/// master cell group is the one the UE is anchored on, the secondary cell group the
/// one a dual-connectivity secondary node serves. A bearer with both is a split
/// bearer.
enum CellGroup
{
    MCG = 0;
    SCG = 1;
    UNKNOWN_CELL_GROUP = 2;
}

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