I think it's literally the ability check referenced by the assist ability check option in the help actionSeems to me this isn't using an ability with a skill but just an ability. What does the proposed rule say about that?
I wrote an adventure (in another system) in which the heroes had a climactic escape at the main gate and had to fight off attackers who kept coming every round, while trying to turn a windlass to raise a portcullis. It explicitly used the strength score (or its equivalent in this system) and allowed for two people to cooperate. Of course, you can't really fight off guards while turning a windlass.
ABILITY CHECK
The Ability Check is one of three types of d20
Tests. The rules often call for an Ability Check,
and the DM can also call for an Ability Check,
determining which ability to use when a creature
attempts something (other than an Attack Roll or
a Saving Throw) that has a chance of meaningful
failure. When the outcome is uncertain and
narratively interesting, the dice determine the
results.
The Ability Check has the following special
rules.
SKILLS
When you make an Ability Check, the rules or the
DM determines whether a Skill Proficiency is
relevant to the check. If you have a relevant Skill
Proficiency, you can add your Proficiency Bonus
to the roll. For example, if a rule refers to a
Strength Check (Acrobatics or Athletics), you can
add your Proficiency Bonus to the check if you
have Acrobatics or Athletics Proficiency.
ACTION REQUIRED
Making an Ability Check requires you to take an
Action unless a rule says otherwise. Several of
the named Actions—such as Hide and
Influence—include Ability Checks.
The DM may override this requirement and
allow a particular Ability Check to be made as
part of a Bonus Action or as no Action at all.
DIFFICULTY CLASS
The DM determines the Difficulty Class of an
Ability Check and can override a DC specified in
the rules. The Typical Difficulty Class table
shows the most common DCs.
The default DC for a check is 15, and it is rarely
worth calling for an Ability Check if the DC is as
low as 5, unless the potential failure is
narratively interesting.
TYPICAL DIFFICULTY CLASS
Task Difficulty DC
Very Easy 5
Easy 10
Medium 15
Hard 20
Very Hard 25
Nearly Impossible 30
The Ability Check is one of three types of d20
Tests. The rules often call for an Ability Check,
and the DM can also call for an Ability Check,
determining which ability to use when a creature
attempts something (other than an Attack Roll or
a Saving Throw) that has a chance of meaningful
failure. When the outcome is uncertain and
narratively interesting, the dice determine the
results.
The Ability Check has the following special
rules.
SKILLS
When you make an Ability Check, the rules or the
DM determines whether a Skill Proficiency is
relevant to the check. If you have a relevant Skill
Proficiency, you can add your Proficiency Bonus
to the roll. For example, if a rule refers to a
Strength Check (Acrobatics or Athletics), you can
add your Proficiency Bonus to the check if you
have Acrobatics or Athletics Proficiency.
ACTION REQUIRED
Making an Ability Check requires you to take an
Action unless a rule says otherwise. Several of
the named Actions—such as Hide and
Influence—include Ability Checks.
The DM may override this requirement and
allow a particular Ability Check to be made as
part of a Bonus Action or as no Action at all.
DIFFICULTY CLASS
The DM determines the Difficulty Class of an
Ability Check and can override a DC specified in
the rules. The Typical Difficulty Class table
shows the most common DCs.
The default DC for a check is 15, and it is rarely
worth calling for an Ability Check if the DC is as
low as 5, unless the potential failure is
narratively interesting.
TYPICAL DIFFICULTY CLASS
Task Difficulty DC
Very Easy 5
Easy 10
Medium 15
Hard 20
Very Hard 25
Nearly Impossible 30
It's all very clear from what I read. I think the disagreement is largely a matter if the GM should be allowed to say no to unreasonable actions & have the rules support them or if the GM should need to change create & nullify rules in order to say no to unreasonable or poorly formed ideas that may or may not be an action depending on consensus.