The ‘Alignment level’ in an audio signal chain or on an audio recording is an anchor point that represents a reasonable or typical level. It does not represent a particular sound level or signal level or digital representation, but it can be defined as corresponding to particular levels in each of these domains.
For example, alignment level is commonly 0dBu in broadcast chains in places where the signal exists as analogue voltage. It most commonly is at -18dB FS (18dB below full scale digital) on digital recordings for programme exchange, in accordance with European Broadcasting Union (EBU) recommendations. 24-bit original or master recordings commonly have alignment level at -24dB FS in order to allow extra headroom, which can then be reduced to match the available headroom of the final medium by Audio level compression. FM broadcasts usually have only 9dB of headroom as recommended by the EBU.
Using alignment level rather than maximum permitted level as the reference point allows more sensible headroom management throughout the audio chain, so that quality is only sacrificed through level compression as late as possible.