Settlement refers to the transfer of funds and the title of a security between the buyer and the seller. Equities (stocks, bonds and ETFs) have T+2 settlement whereas options and government securities are T + 1. There are some exceptions.
You are only allowed to trade settled cash in a cash account. That means that with T+2, the funds from a sale will not be available for two days. There is no limit to how many day trades you can make in a cash account as long as you are using settled funds. For example, assuming that you do not pay commissions, if you have $10,000 of settled cash in your account, you can make ten $1,000 purchases in a day. Once done, you will have to wait 2 days until the trades settle and the funds are back in your account.
A margin account is required for certain types of transactions (naked options, leveraging buying, short selling). In a margin account, you can borrow money from your broker to purchase stock. Reg T initial margin for non leveraged equities is 50% though brokers can require higher initial margin. Some securities have higher margin restrictions (leveraged ETFs, stocks below $5).
In the USA, a Pattern Day Trader is defined as a person who executes 4 or more day trades (options and equities) in a rolling FIVE business day period in a margin account, provided the number of day trades are more than six percent of the customer's total trading activity for that same five-day period.
A PDT must maintain a minimum equity of $25k on any day that trades are made. It must be in the account prior to the day trading. If the account falls below $25k, the PDT will not be permitted to day trade until the account is restored to the $25k minimum equity level. You will have at most, 5 business days to deposit funds to meet the call. Until the call is met, day-trading will be restricted to two times maintenance margin excess. If the margin call is not met by the fifth business day, the account will be further restricted to trading only on a cash available basis for 90 days or until the call is met.
A PDT is allowed intraday to trade four times the maintenance margin excess in the account as of the close of business of the previous day but must revert to the standard 50% overnight margin by the end of the current day. Brokers have the right to set more restrictive levels of margin (less than 4:1). In reality, you can't go to 4X because market fluctuations will trigger a margin violation.
While Reg T margin is 50% and PDT intraday margin is 25% (SEC limits), brokers can have higher house requirements and restrict the amount of margin offered.