[Bernardo II]

Sin ir más lejos canales asignados a el AIS
te dejo el extracto donde lo explica

What does “A” or “B” mean in a VHF channel?
VHF channels are labelled with an “A” or “B” when they are a new simplex channel that has been derived from an original duplex channel.
For example, when AIS was developed there was not sufficient space within the VHF spectrum to assign it a new frequency. The only option was to repurpose another channel that was already in use.
You can repurpose a duplex channel and create two simplex channels instead.
In the case of AIS, they started with the two duplex channels, 87 and 88.

Channel Type Ship Frequency (A) Shore Frequency (B)
87 Duplex 157.375 MHz 161.975 MHz
88 Duplex 157.425 MHz 162.025 MHz

If you re-designate them as simplex channels, you free up the shore frequencies that are no longer needed. The result is four new channels.

Channel Type Frequency
87A Simplex 157.375 MHz
88A Simplex 157.425 MHz
87B Simplex 161.975 MHz
88B Simplex 162.025 MHz

87B and 88B were given to AIS and renamed as AIS1 and AIS2.*
The simplex ship frequencies were retained for voice communication and the “A” suffix was dropped to make it simpler for numbering on a VHF set.*
It was only possible to drop the suffix because it no longer acts as a duplex channel anywhere in the world. If it was still used as a duplex channel somewhere, the suffix would need to remain to differentiate between the two.
When you see an “A” or “B” on a VHF channel number, it just means that it was originally one half of a duplex channel.

A resumidas cuenta del canal 87 y 88 sacaron cuastro nuevos canales

Channel Type Frequency
87A Simplex 157.375 MHz
88A Simplex 157.425 MHz
87B Simplex 161.975 MHz
88B Simplex 162.025 MHz

Tambien mucho de estos canales se usaban para conexión telefonica con tierra. con el auge del telefono movil esta radio costeras han ido cerrando

Hay otro ejemplos como uso de avisos meteorologicos y servicio similares , son cosas que ves muy por encima.