It is not classed as anything.
If you read BS6798 it is allowed. Here is a bit from it if you don't have it.
4.3.4.2 Connection to an internal waste pipe (Figure 2)
Provided that the condensate drainage pipe meets the requirements of 4.3.3 and 4.3.4, there is no length restriction.
The connection shall be made to the internal waste pipe from a sink, washing machine or dishwasher. The connection may be made either downstream [Figure 2a)] or upstream [Figure 2b)] of the waste trap.
NOTE If practical, the connection should be made to the upper part of the pipe wall.
— If the connection is downstream of the waste trap and the boiler does not have an integral condensate trap with a seal of at least 75 mm then an additional trap of at least 75 mm shall be fitted. A visible air
break shall be included between these traps.
— If the connection is upstream of the waste trap, then a visible air break is necessary between the waste trap and the condensate trap but, in the case of a sink, this is provided by the sink waste pipe itself as
long as the sink has an integral overflow.
In order to prevent waste from the sink, washing machine or dishwasher entering the condensate trap, there shall be a minimum of 100 mm between the visible air break at the lowest condensate trap and the
top of the sink or visible air break serving the washing machine or dishwasher.
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All the answers are in the books if you read them.