= Preset tare device
A \b[preset tare device] subtracts a <preset tare value> from a measured gross or net indication and displays the calculated result.<Non-automatic weighing instruments, Part 1>[OIML R76-1:2006(E), T.2.7.5, printed p12; T.5.3.1–T.5.3.2, printed p16]{ref} A stored container value can therefore be reused without weighing each empty container individually.
Suppose a container and its contents have measured <gross mass> $G=2.30\,\mathrm{kg}$ and the entered <preset tare value> is $PT=0.30\,\mathrm{kg}$. Subtraction gives a calculated <net mass> of $2.00\,\mathrm{kg}$. If this particular container actually has <tare mass> $T=0.34\,\mathrm{kg}$, however, its contents have <mass> $1.96\,\mathrm{kg}$. Even an otherwise exact gross measurement would then overestimate the contents by $0.04\,\mathrm{kg}$.
The error follows without any instrument-specific assumption:
$$
N_{\rm calc}=G-PT,\qquad N_{\rm true}=G-T,
\qquad N_{\rm calc}-N_{\rm true}=T-PT.
$$
A mismatch in the entered container value passes directly into the result. Reusing one value for nominally identical containers is therefore an assumption about their <tare masses>, rather than a new measurement of each container.
The starting indication also matters. If an ordinary <tare device> has already removed a $0.30\,\mathrm{kg}$ container contribution, the displayed $2.00\,\mathrm{kg}$ is already net for that reference. Subtracting the same $0.30\,\mathrm{kg}$ again would give $1.70\,\mathrm{kg}$. A deliberate second subtraction can represent another distinct component, but applying the same container correction twice is a different calculation from removing its <tare mass> once.
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