Most sims use a 'Yellow Flag' warning system, which helps to notify competitors of incidents ahead of them on track. For some sims, it may be a Flag Icon on the screen (rFactor top left).
For others (GPL), the actual Marshals are waving the flags. Either way, if a Yellow Flag warning is present, then please try to react according to standard Motorsport rules, such as being shown to drivers to warn them to slow down and not to over take other cars because there is a dangerous situation ahead.
RCR 17a/. A single waved yellow flag means slow down and warns of danger ahead.
RCR 17b/. A single waved yellow flag means overtaking is prohibited.
Consequence for non-adherence:
A Competitor who doesn't react/slow sufficiently to enable them to react to trouble ahead, which results in further incident may incur a warning/penalty. A Competitor who reacts in such a manner so as to create further incident (e.g. Sudden change of course, Sudden reduction in speed, relative to cars around them), may receive a warning. A Competitor who clearly uses the Yellow Flag period as an opportunity to pass other cars may incur a warning/penalty. Penalties will be applied depending on the level of the incident and at the discretion of Admin. These may range from Lap Penalties, to Time Penalties, to Drive Through Penalties, to Grid Penalties.
In Sim-Racing, this rule is tougher to police than in real racing. Only if someone blatently disregards a dangerous situation and makes multiple passes or causes avoibale damage through a lack of caution would a full penalty be given. In most caes, it would be a warnning.