Short answer. Section 2 of Rule 5 clarifies that 'Municipal Trial Courts,' as used throughout the Rules, is an umbrella term covering Metropolitan Trial Courts, Municipal Trial Courts in Cities, Municipal Trial Courts, and Municipal Circuit Trial Courts — all four first-level court types.
What the law says
The term "Municipal Trial Courts" as used in these Rules shall include Metropolitan Trial Courts, Municipal Trial Courts in Cities, Municipal Trial Courts, and Municipal Circuit Trial Courts.
Rule 5, Section 2 — Meaning of terms. Read the full provision →
Why a definitional section is needed
The Philippine judiciary's first level of courts is not a single, uniformly named entity — it includes several distinct designations depending on the locality and structure of the jurisdiction: Metropolitan Trial Courts in Metro Manila, Municipal Trial Courts in Cities, plain Municipal Trial Courts, and Municipal Circuit Trial Courts covering multiple municipalities. Section 2 exists to keep the Rules from having to spell out all four every time a provision refers to first-level courts.
What the shorthand covers
Section 2 states plainly that the term 'Municipal Trial Courts,' as used in the Rules, shall include all four of these court types. This means any provision elsewhere in the Rules of Civil Procedure that refers to the Municipal Trial Courts is, by this definition, referring collectively to Metropolitan Trial Courts, MTCs in Cities, ordinary MTCs, and Municipal Circuit Trial Courts, unless the specific context indicates otherwise.
Practical effect for a litigant
A litigant filing in, say, a Metropolitan Trial Court in Quezon City can rely on provisions of the Rules that speak generally of the 'Municipal Trial Courts' — those provisions apply to their court too, because of this section's inclusive definition. There is no need to search for a court-type-specific provision when the general Municipal Trial Courts language already reaches every first-level court under Section 2's definition. It also spares drafters of pleadings, motions, and even the Rules themselves from having to repeat all four court names every time a provision needs to speak of the first level of the judicial hierarchy generally.
What falls outside the definition
The definition in Section 2 covers only the first-level courts named in the section itself; it says nothing about Regional Trial Courts, the Court of Appeals, or the Supreme Court, which sit above the first level and are addressed elsewhere in the Rules. A reference elsewhere in the Rules to 'Regional Trial Courts' is not swept into this Section 2 definition, since the section's inclusive language is confined to the four first-level court types it actually lists. The term does not reach the Regional Trial Courts, the Court of Appeals, or the Supreme Court, none of which are first-level courts; Section 2's inclusive list is confined strictly to the four first-level designations it actually names.