If you don't understand the law when filling in the PIP form, then you will not fill it in correctly.
If you fill it in incorrectly, then you have no clear documentary record to criticise a most basic flaw in the assessment.
In other words, if you can't walk a distance, most of the time at over half the speed of a normal person, you may be legally unable to do it, and should tick 'no' against that distance, and then explain why.
The same (for PIP) is the case for all of the activities. This does not mean that all assessments are correct, but at least you're not starting from behind as if the form was incorrect, and you answer it according to the distances you can do once on a good day, say.
I understood the law, I filled in the form correctly, I used the appropriate language and supplied relevant additional information in the form of 30+ pages of detailed answers breaking down what I could do, how often, what the effects were, what I couldn't do etc. etc., with statements from others, documents from my local council etc. I then went to (was driven to and assisted with) a PIP assessment and again gave them information in response to the questions asked. The paramedic who assessed me had no understanding of asperger's or M.E. and, on the basis of an "observational mental state exam" gave me zero points in all categories apart from how far i could walk, giving me 4 points for 50-200m when it had been clearly stated that I couldn't walk more than 30m at a time before having to stop due to the level of pain and disorientation, using crutches, and couldn't do this repeatedly, safely or reliably without being escorted etc. etc - IMO I suspect she was in a hurry when completing the medical program and fully intended to give me zero points but mis clicked, the 4 points being the next box down.
Obviously I obtained a copy of this report, the assessor appears to have deliberately lied, given incomplete information, completely omitted relevant information she was told etc. Several tests mentioned did not occur, other tests which did occur that would indicate significant issues were not mentioned. She appears to have deliberately set out to give the DWP decision maker the impression I was lying, and by a series of misdirections throughout the report apparently succeeded.
The DWP ignored my medical evidence, my PIP2 form, my supplementary evidence, everything, apart from the PIP assessment report, completed by someone who had clearly been well trained to minimise the impact of disability.
So....anything more to say about my alleged incompetence in dealing with a perfectly fair and well balanced system or are you prepared to accept that the actual implementation doesn't match the law, and based on my experience and the experiences of thousands of others is designed to not match the law, is deliberately designed to be unfair.
edit...BTW I have submitted a MR request, comprising a 24 page document, which both points out the flaws in the assessment report and restates why I think I should be entitled to points in each category. This has a 13% chance of succeeding (at all, no idea what the stats are for getting the correct award), and when it fails I will attempt to prepare an additional submission for appeal - which has on average a 62% chance of succeeding (again I have no idea how this breaks down re oral vs paper hearing). End result is likely to be I win, but after having had a considerable amount of stress, being much more ill than I needed to be for months, putting several other people to serious inconvenience, and the loss of 60% of my income for several months (with the loss of DLA I also lose severe disability premium - in total a loss of just under £140 a week - yes I "may" get it back, eventually, but how do I live in the meantime?) All of this because PIP assessors are trained to minimise disability and DWP decision makers generally just follow exactly what the assessment report says.