In a major change from the previous edition (IEC 60601-2-51:2003), this standard tests the Goldberger and Wilson LEAD network using CAL waveforms only. There are some concerns with the standard which are outlined below:

the standard does not indicate if the tests must be performed by analogue means, or if optionally digital tests are allowed as indicated in other parts of the standard. It makes sense to apply the test in analogue, as there is no other test in the standard which verifies the basic accuracy of sensitivity for the complete system (analogue and digital).
The CAL (and ANE) signals are designed in a way that RA is the reference ground (in the simulation data, RA is always zero; in the circuit recommended in IEC 60601-2-51, RA is actually connected to ground). This means that an error on RA cannot be detected by CAL or ANE signals. The previous standard was careful to test all leads individually, including cases where a signal is provided only to RA (other leads are grounded), ensuring errors on any individual lead would be detected.
The allowable limit is 10%. This is a relaxation from IEC 60601-2-51, conflicts with the requirement statement in Clause 201.12.4.102.3.1 and also requirements for voltage measurements in Clause 201.12.1.101.2, all of which use 5%. Furthermore, many EMC tests refer to using CAL waveforms with the criteria from Clause 201.12.1.101.2 (5%), not the 10% which comes from this clause.

A limit of 5% makes sense for diagnostic ECGs and is not difficult with modern electronics and historically has not been an issue. There is no explanation where the 10% comes; at a guess the writers may have trying to separate basic measurement sensitivity (5%) from the network accuracy (5%). In practice, it makes little sense to separate these out as ECGs don't provide access to the raw data from each lead electrode, only the final result which includes both the sensitivity and the network. As such we can only evaluate the complete system based on inputs (lead electrodes, LA, LL, RA etc) and outputs (displayed LEAD I, II, III etc).

As mentioned above, there is no other test in IEC 60601-2-25 which verifies the basic sensitivity of the ECG. Although sensitivity errors may become apparent in other tests, it makes sense to establish this first as a system, including the weighting network, before proceeding with other tests. While modern ECGs, from quality manufacturers and designed specifically for for diagnostic work generally have little problem for 5%, experience indicates that lower quality manufacturers and in particular multipurpose devices (e.g. patient monitor with diagnostic functions) can struggle to meet basic accuracy requirement for sensitivity.