A RecId is a unique identifier for any record in:
a table (for DAX 4.0)
the entire company (DAX 3.0 and earlier)
RecId is assigned to every record by the kernel of Dynamics AX each time a
record is created.
an ID is not a standard name in DAX, so it's either a customized field added
into the table you are looking into, or just an identifier of an object.
For example, there is a TABLE ID, which is the number of each table in DAX.
A FIELD ID is an integer identifing a field in a table.
You can make your IDs string as well, for example the NumberSequences work