If you are asking what judges use for guidance when they are making their decisions, there are several answers, and it will depend upon the proceeding and the type of judge and court.
First, all judges MUST follow/consider:
- Relevant Statutes and Codes
- Relevant Regulations; and
- Binding Precedent (previous judicial opinions) that directly applies to the case at bar.
Whether a statute is "relevant" or a prior case is "binding" will depend upon the jurisdiction (where the court is) and what type of court it is (trial, intermediate appellate, or highest). Lower courts, for example, must follow the decisions of higher (appellate) courts in their jurisdiction that are directly relevant to their case.
However, it is not always clear whether a statute, regulation, or case is "relevant." This is because the meaning of the statute, code, or regulation can be ambiguous. Also, the "holding" of the prior cases (what rule it actually stands for), can be very unclear.
Accordingly, to interpret the meaning of a statute, judges will often look (for guidance) to:
- The dictionary (to interpret the plain text of the statute),
- Cannons of statutory construction (understood rules that judges follow when discerning the meaning of statutes -- for example, the "rule of lenity" provides that if a criminal statute is ambiguous, the judge should favor the defendant),
- How other judges have interpreted the case, statute or regulation (even judges not in that jurisdiction),
- The legislative history (reports of what the legislature was thinking when they enacted the statute),
- Opinions of experts (to interpret technical or trade terms in the statute),
- Unofficial administrative agency opinions (similar to experts), and
- The circumstances in which the statute was written (to discern its purpose)
Again, judges may look for "guidance" to non-binding judicial opinions, they also look to "policy considerations," or what affect their opinion may have on the public at large.
In criminal cases, judges follow sentencing guidelines to give the appropriate sentence.
If the case is on appeal from an administrative decision or a lower court, judges may be required to give "deference" to the lower court's decision, and consider the lower court's reasoning in deciding the outcome of the case.
In short, judges seek guidance from several different sources. Attorneys play a large role in this as well by submitting briefs to the judge (often containing arguments similar to the ones given above), which the judge usually considers fully before making a decision.